How to be a Performance Detective - Performance.now() Conference 2022

When starting on a web performance journey, many companies’ first step is to start monitoring performance metrics over time. But the second step is often much harder - now that you have all of this data, what do you do when you see a regression? How do you sift through the clues to connect a change in a graph to the code that engineers ship to production? How do you become a performance detective?

In this session, Katie will crack open her casefiles to share some real-world examples where performance metrics changed for the worse, and walk through how to find the culprit. You’ll learn what clues to look for to understand a regression in your Core Web Vitals, how different performance metrics influence each other, and when to use data gathered from real users vs. synthetic tests in the lab.

Transcript

1. DETECTIVE
   DETECTIVE
   H O W T O B E A
   E
   P
   o
   FRM Nc
   R
   e
   a
   Katie Sylor-Miller
   Frontend Architect, Etsy
   Fairy Gitmother, OhShitGit.com
   @ksylor
   Detective Pikachu © 2019 Warner Bros., Pokémon © 2022 Pokémon. © 1995–2022 Nintendo/Creatures Inc./GAME FREAK inc.
   

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2. WHY DID THIS
   PERFORMANCE
   METRIC GET WORSE?
   

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3. HARD TRUTH #1
   EVERY ADDITIONAL
   BYTE WILL SLOW
   METRICS DOWN
   

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4. HARD TRUTH #2
   IT DEPENDS
   

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5. HARD TRUTH #3
   THERE'S NO ONE RIGHT
   WAY TO MEASURE
   PERFORMANCE
   

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6. PERFORMANCE METRIC TYPES
   Icons By Andreuvv - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=90073970
   @ksylor
   

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7. PERFORMANCE METRIC TYPES
   WHAT
   HOW WHERE
   Primary
   Diagnostic
   Timeline-based
   Independent
   Field/Real User
   Lab/Synthetic
   @ksylor
   

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8. PERFORMANCE METRIC TYPES
   HOW ARE THEY MEASURED?
   Timeline-based Independent
   vs
   @ksylor
   

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9. PERFORMANCE METRIC TYPES - TIMELINE-BASED VS. INDEPENDENT
   TIMELINE-BASED METRICS
   Start a timer at 0, log a timeline of events as the page loads
   Navigation
   Start TTFB FCP LCP DCL TTI Page Load
   0ms 200ms 300ms 400ms 500ms 600ms 700ms
   @ksylor
   

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10. PERFORMANCE METRIC TYPES - TIMELINE-BASED VS. INDEPENDENT
    TIMELINE-BASED METRICS
    Start a timer at 0, log a timeline of events as the page loads
    Early slowdowns impact later metrics
    TTFB FCP TTI Page Load
    0ms 200ms 300ms 400ms 500ms 600ms 700ms
    TTFB FCP TTI Page Load
    0ms 200ms 300ms 400ms 500ms 600ms 700ms 800m
    @ksylor
    LCP DCL
    LCP DCL
    Navigation
    Start
    Navigation
    Start
    

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11. PERFORMANCE METRIC TYPES - TIMELINE-BASED VS. INDEPENDENT
    INDEPENDENT METRICS
    Events that are measured independent of the loading timeline
    0ms 200ms 300ms 400ms 500ms 600ms 700ms
    150ms FID / INP / TBT
    @ksylor
    

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12. PERFORMANCE METRIC TYPES - TIMELINE-BASED VS. INDEPENDENT
    INDEPENDENT METRICS
    Events that are measured independent of the loading timeline
    150ms FID / INP / TBT
    0ms 200ms 300ms 400ms 500ms 600ms 700ms
    0.05 Layout Shift 0.125 Layout Shift
    0ms 200ms 300ms 400ms 500ms 600ms 700ms
    0.175 CLS
    @ksylor
    

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13. PERFORMANCE METRIC TYPES
    WHERE ARE THEY MEASURED?
    Field/Real User Lab/Synthetic
    vs
    @ksylor
    

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14. PERFORMANCE METRIC TYPES - FIELD/REAL USER VS. LAB/SYNTHETIC
    THE METRIC VENN DIAGRAM
    Field/Real User Lab/Synthetic
    Icons By Andreuvv - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=90073970
    @ksylor
    FID
    TBT
    TTFB
    TTI
    Speed
    Index
    FCP
    LCP
    CLS
    DCL
    INP
    Page
    Load
    CPU
    Usage
    Start
    Render
    Resource
    Timings
    User
    Timings
    

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15. PERFORMANCE METRIC TYPES - FIELD/REAL USER VS. LAB/SYNTHETIC
    REAL USER MONITORING (RUM)
    ✦ Collect data from Real User sessions via browser-exposed APIs
    ✦ Available via:
    ✦ Third-party monitoring services
    ✦ Public datasets Chrome User Experience (CrUX), Akamai RumArchive
    ✦ Open source JS libraries
    ✦ or Do-It-Yourself
    @ksylor
    

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16. PERFORMANCE METRIC TYPES - FIELD/REAL USER VS. LAB/SYNTHETIC
    REAL USER MONITORING (RUM)
    ✦ Cool stuff:
    ✦ Captures the full story of your real
    users' experiences
    ✦ Interaction-based metrics like FID
    and INP are only available in RUM
    @ksylor
    

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17. PERFORMANCE METRIC TYPES - FIELD/REAL USER VS. LAB/SYNTHETIC
    REAL USER MONITORING (RUM)
    ✦ Cool stuff:
    ✦ Captures the full story of your real
    users' experiences
    ✦ Interaction-based metrics like FID
    and INP are only available in RUM
    ✦ Meh stuff:
    ✦ Browser support lacking for some
    metrics
    ✦ Every data point is a tradeoff
    ✦ Good for trends, not in-depth
    analysis
    @ksylor
    

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18. PERFORMANCE METRIC TYPES - FIELD/REAL USER VS. LAB/SYNTHETIC
    RUM POPULATIONS
    ✦ Performance data is variable and
    follows a log-normal distribution
    ✦ Cumulative Distribution Function (CDF)
    ✦ How do you compare a continuum of
    populations? Industry standard of p75
    @ksylor
    

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19. PERFORMANCE METRIC TYPES - FIELD/REAL USER VS. LAB/SYNTHETIC
    SYNTHETIC
    ✦ Collect data from a page load in controlled conditions
    ✦ Available via:
    ✦ Webpagetest.org or Lighthouse
    ✦ In the browser or programmatically
    ✦ Continuous monitoring or in-the-moment
    @ksylor
    

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20. PERFORMANCE METRIC TYPES - FIELD/REAL USER VS. LAB/SYNTHETIC
    SYNTHETIC
    ✦ Cool stuff:
    ✦ Apples-to-apples comparison for
    diagnosis
    ✦ In-depth metrics not easily
    available from RUM
    ✦ Available in pre-prod
    @ksylor
    

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21. PERFORMANCE METRIC TYPES - FIELD/REAL USER VS. LAB/SYNTHETIC
    SYNTHETIC
    ✦ Cool stuff:
    ✦ Apples-to-apples comparison for
    diagnosis
    ✦ In-depth metrics not easily
    available from RUM
    ✦ Available in pre-prod
    ✦ Meh stuff:
    ✦ Not every metric is available, but
    there are correlated metrics (e.g.
    TBT or TTI instead of FID or INP)
    ✦ Doesn't capture interactions
    @ksylor
    

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22. PERFORMANCE METRIC TYPES
    WHAT DO THEY MEASURE?
    Primary Diagnostic
    vs
    @ksylor
    

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23. PERFORMANCE METRIC TYPES - PRIMARY VS. DIAGNOSTIC
    PRIMARY METRICS - WHAT IS MOST IMPORTANT TO USERS?
    Icons By Andreuvv - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=90073970
    @ksylor
    LCP
    Largest Contentful Paint
    FID
    First Input Delay
    CLS
    Cumulative Layout Shift
    

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24. PERFORMANCE METRIC TYPES - PRIMARY VS. DIAGNOSTIC
    PRIMARY METRICS - WHAT IS MOST IMPORTANT TO YOUR USERS?
    LCP
    Largest Contentful Paint
    FID
    First Input Delay
    CLS
    Cumulative Layout Shift
    FCP
    First Contentful Paint
    ???
    Your Important Metric
    TTFB
    Time to First Byte
    @ksylor
    Icons By Andreuvv - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=90073970
    

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25. PERFORMANCE METRIC TYPES - PRIMARY VS. DIAGNOSTIC
    DIAGNOSTIC METRICS - WHAT CAUSED A CHANGE?
    Navigation Timing Resource Timing LCP/CLS Attribution
    Synthetic-only Metrics
    Long Task Timing Custom Timings
    Icons By Andreuvv - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=90073970
    @ksylor
    

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26. PERFORMANCE METRIC TYPES - PRIMARY VS. DIAGNOSTIC
    DIAGNOSTIC METRICS - NAVIGATION TIMING
    ✦ E.g. the old school
    metrics
    ✦ Before Fetch
    ✦ Connection Time
    ✦ Time to First Byte
    ✦ DOM Content Loaded
    ✦ Page Load
    Image via MDN https://developer.mozilla.org/en-US/docs/Learn/Performance/Measuring_performance
    @ksylor
    

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27. PERFORMANCE METRIC TYPES - PRIMARY VS. DIAGNOSTIC
    DIAGNOSTIC METRICS - RESOURCE TIMING IN SYNTHETIC
    ✦ What resources
    loaded?
    ✦ In what order?
    ✦ How big were
    they?
    @ksylor
    

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28. PERFORMANCE METRIC TYPES - PRIMARY VS. DIAGNOSTIC
    DIAGNOSTIC METRICS - RESOURCE TIMING IN RUM
    Track the count and sizes of all
    resources:
    @ksylor
    ✦ HTML
    ✦ CSS
    ✦ JS
    ✦ Images
    ✦ Videos
    ✦ Fonts
    ✦ XHR requests
    

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29. PERFORMANCE METRIC TYPES - PRIMARY VS. DIAGNOSTIC
    DIAGNOSTIC METRICS - LCP/CLS ATTRIBUTION IN SYNTHETIC
    @ksylor
    ✦ Which elements
    shifted?
    ✦ What was the
    LCP element?
    

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30. PERFORMANCE METRIC TYPES - PRIMARY VS. DIAGNOSTIC
    DIAGNOSTIC METRICS - LCP/CLS ATTRIBUTION IN RUM
    @ksylor
    

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31. PERFORMANCE METRIC TYPES - PRIMARY VS. DIAGNOSTIC
    DIAGNOSTIC METRICS - LONG TASKS IN SYNTHETIC
    ✦ How many Long Tasks?
    ✦ When did they start?
    @ksylor
    

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32. PERFORMANCE METRIC TYPES - PRIMARY VS. DIAGNOSTIC
    DIAGNOSTIC METRICS - LONG TASKS IN RUM
    ✦ Log count and duration
    ✦ Sum of duration = proxy for TBT
    ✦ Attribution is per container so
    useful only for 1st vs. 3rd party
    tracking but not much else
    @ksylor
    

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33. PERFORMANCE METRIC TYPES - PRIMARY VS. DIAGNOSTIC
    DIAGNOSTIC METRICS - CUSTOM TIMINGS
    ✦ Make your own timings!
    ✦ Webpagetest will report your
    custom timings in the UI
    ✦ Full cross-browser support
    @ksylor
    

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34. PERFORMANCE METRIC TYPES - PRIMARY VS. DIAGNOSTIC
    DIAGNOSTIC METRICS - SYNTHETIC-ONLY METRICS
    ✦ JS Execution details
    ✦ Total blocking time (TBT)
    ✦ Time to Interactive (TTI)
    ✦ CPU usage
    ✦ Processing breakdown (script
    parse/eval/render/layout/paint
    etc.)
    @ksylor
    

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35. PERFORMANCE METRIC TYPES - PRIMARY VS. DIAGNOSTIC
    DIAGNOSTIC METRICS - SYNTHETIC-ONLY METRICS
    ✦ JS Execution details
    ✦ Total blocking time (TBT)
    ✦ Time to Interactive (TTI)
    ✦ CPU usage
    ✦ Processing breakdown (script
    parse/eval/render/layout/paint
    etc.)
    ✦ Rendering details
    ✦ Filmstrip screenshots
    ✦ Start Render
    ✦ Speed Index
    ✦ Visually Complete
    @ksylor
    

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36. TIME TO CATCH
    SOME PERF METRICS!
    @ksylor
    

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37. HTTPS://REPLICATE.COM/LAMBDAL/TEXT-TO-POKEMON
    @ksylor
    

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38. Emcee and tall person
    Tim Kadlec TRAINER
    Emcee and running enthusiast
    Henri Helvetica TRAINER
    HTTPS://REPLICATE.COM/LAMBDAL/TEXT-TO-POKEMON
    @ksylor
    

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39. Timeline-based
    Primary - Core Web Vitals
    RUM and Synthetic
    Largest Contentful Paint
    LCP
    CASE FILES
    @ksylor
    

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40. CASE FILES: LARGEST CONTENTFUL PAINT (LCP)
    WHO KILLED LCP?
    @ksylor
    

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41. CASE FILES: LARGEST CONTENTFUL PAINT (LCP)
    BEFORE (OFF) AFTER (ON)
    Aha! The design of the likely
    LCP element changed!
    @ksylor
    

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42. CASE FILES: LARGEST CONTENTFUL PAINT (LCP)
    DIAGNOSING CHANGES TO LCP
    1. Did the LCP element change?
    @ksylor
    

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43. TRAINER
    WEBPAGETEST
    TRAINER CARD
    Use to run synthetic tests on a real
    browser and device
    Webpagetest.org @ksylor
    

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44. CASE FILES: LARGEST CONTENTFUL PAINT (LCP)
    GOTCHA!
    This is not the
    LCP element
    you are
    looking for
    @ksylor
    

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45. TAKE 2
    The LCP Element
    did not change
    CASE FILES: LARGEST CONTENTFUL PAINT (LCP) @ksylor
    

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46. DIAGNOSING CHANGES TO LCP
    1. Did the LCP element change? NO
    2. Did some characteristic of the LCP element change?
    - If image or video, did the asset size get bigger?
    - (If text, did the font or text change?)
    CASE FILES: LARGEST CONTENTFUL PAINT (LCP) @ksylor
    

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47. BEFORE (OFF) AFTER (ON)
    CASE FILES: LARGEST CONTENTFUL PAINT (LCP)
    Smaller
    Image size
    @ksylor
    

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48. BEFORE (OFF) AFTER (ON)
    CASE FILES: LARGEST CONTENTFUL PAINT (LCP)
    Smaller
    Image size
    @ksylor
    Hint to the
    real culprit!
    

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49. DIAGNOSING CHANGES TO LCP
    1. Did the LCP element change? NO
    2. Did some characteristic of the LCP element change? NO
    - If image or video, did the asset size get bigger?
    - (If text, did the font or text change?)
    3. Did metrics before LCP in the timeline change?
    - e.g. TTFB, FCP, DCL?
    CASE FILES: LARGEST CONTENTFUL PAINT (LCP) @ksylor
    

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50. RUM
    TRAINER CARD
    Collect data from real user
    sessions
    Real User Monitoring TRAINER @ksylor
    

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51. Timeline-based
    Primary (not CWV)
    RUM and Synthetic
    Time to First Byte
    TTFB
    SUSPECT #1
    @ksylor
    

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52. Timeline-based
    Primary (not CWV)
    RUM and Synthetic
    Time to First Byte
    TTFB
    CASE FILES
    @ksylor
    

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53. Timeline-based
    Primary (not CWV)
    RUM and Synthetic
    First Contentful Paint
    FCP
    SUSPECT #2
    @ksylor
    

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54. Timeline-based
    Primary (not CWV)
    RUM and Synthetic
    First Contentful Paint
    FCP
    CASE FILES
    @ksylor
    

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55. DOM Content Loaded
    DCL
    SUSPECT #3
    Timeline-based
    Diagnostic
    RUM and Synthetic
    @ksylor
    

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56. DOM Content Loaded
    DCL
    CASE FILES
    Timeline-based
    Diagnostic
    RUM and Synthetic
    @ksylor
    

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57. Page Load
    PAGE LOAD
    SUSPECT #4
    Timeline-based
    Diagnostic
    RUM and Synthetic
    @ksylor
    

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58. Page Load
    PAGE LOAD
    CASE FILES
    Timeline-based
    Diagnostic
    RUM and Synthetic
    @ksylor
    

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59. DIAGNOSING CHANGES TO LCP
    1. Did the LCP element change? NO
    2. Did some characteristic of the LCP element change? NO
    - If image or video, did the asset size change?
    - (If text, did the font or text change?)
    3. Did metrics before LCP in the timeline change? NO
    - e.g. TTFB, FCP, DCL?
    4. Did the loading order change?
    CASE FILES: LARGEST CONTENTFUL PAINT (LCP) @ksylor
    

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60. BEFORE (OFF)
    AFTER (ON)
    LCP Element #12
    CASE FILES: LARGEST CONTENTFUL PAINT (LCP) @ksylor
    

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61. BEFORE (OFF)
    AFTER (ON)
    LCP Element #12
    LCP Element #13
    A wild CSS file appears!
    CASE FILES: LARGEST CONTENTFUL PAINT (LCP) @ksylor
    

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62. DIAGNOSING CHANGES TO LCP
    1. Did the LCP element change? NO
    2. Did some characteristic of the LCP element change? NO
    - If image or video, did the asset size change?
    - (If text, did the font or text change?)
    3. Did metrics before LCP in the timeline change? NO
    - e.g. TTFB, FCP, DCL?
    4. Did the loading order change? YES!
    5. Did render-blocking assets grow in size or number?
    CASE FILES: LARGEST CONTENTFUL PAINT (LCP) @ksylor
    

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63. CHECK YOUR REQUESTS
    CASE FILES: LARGEST CONTENTFUL PAINT (LCP)
    +1 request Double the bytes :(
    @ksylor
    

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64. DIAGNOSING CHANGES TO LCP
    1. Did the LCP element change? NO
    2. Did some characteristic of the LCP element change? NO
    - If image or video, did the asset size change?
    - (If text, did the font or text change?)
    3. Did metrics before LCP in the timeline change? NO
    - e.g. TTFB, FCP, DCL?
    4. Did the loading order change? YES!
    5. Did render-blocking assets grow in size or number? YES!
    CASE FILES: LARGEST CONTENTFUL PAINT (LCP) @ksylor
    

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65. Independent
    Primary - Core Web Vitals
    Cumulative Layout Shift
    CLS
    CASE FILES
    RUM and Synthetic
    @ksylor
    

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66. CASE FILES: CUMULATIVE LAYOUT SHIFT (CLS)
    WHO KILLED CLS?
    @ksylor
    

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67. BEFORE (OFF) AFTER (ON) Aha! A new module
    above the fold
    CASE FILES: CUMULATIVE LAYOUT SHIFT (CLS) @ksylor
    

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68. DIAGNOSING CLS
    1. What elements are shifting?
    A. Font loading FOUT/FOIT?
    B. Images without dimensions?
    C. Lazy-loaded content without a correct height placeholder?
    D. Injected banners, ads or iframes?
    CASE FILES: CUMULATIVE LAYOUT SHIFT (CLS) @ksylor
    

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69. LIGHTHOUSE
    TRAINER CARD
    Use to do a quick in-browser
    synthetic test
    Lighthouse TRAINER @ksylor
    

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70. BEFORE (OFF) AFTER (ON)
    CASE FILES: CUMULATIVE LAYOUT SHIFT (CLS) @ksylor
    

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71. BEFORE (OFF) AFTER (ON)
    CASE FILES: CUMULATIVE LAYOUT SHIFT (CLS) @ksylor
    

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72. ZOOMING IN ON AFTER (ON)
    CASE FILES: CUMULATIVE LAYOUT SHIFT (CLS)
    Everything
    under the new
    module
    @ksylor
    

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73. WHAT HAPPENED?
    CASE FILES: CUMULATIVE LAYOUT SHIFT (CLS) @ksylor
    

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74. DIAGNOSING CLS
    1. What elements are shifting?
    A. Font loading FOUT/FOIT? NO
    B. Images without dimensions? NO
    C. Lazy-loaded content without a correct height placeholder? YES!
    D. Injected banners, ads or iframes? NO
    CASE FILES: CUMULATIVE LAYOUT SHIFT (CLS) @ksylor
    

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75. Independent
    Primary - Core Web Vitals
    RUM
    First Input Delay
    FID
    CASE FILES
    @ksylor
    

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76. CASE FILES: FIRST INPUT DELAY (FID)
    WHO KILLED FID?
    @ksylor
    

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77. AFTER (ON)
    This new
    button
    loads
    CASE FILES: FIRST INPUT DELAY (FID) @ksylor
    

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78. DIAGNOSING CHANGES TO FID
    1. Is there more Javascript execution, e.g. Long Tasks?
    CASE FILES: FIRST INPUT DELAY (FID) @ksylor
    

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79. Long Tasks
    LONG TASKS
    SUSPECT #1
    Timeline-based
    Diagnostic
    RUM and Synthetic
    @ksylor
    

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80. MORE LONG TASKS MORE PROBLEMS
    CASE FILES: FIRST INPUT DELAY (FID) @ksylor
    

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81. DIAGNOSING CHANGES TO FID
    1. Is there more Javascript execution, e.g. Long Tasks? YES!
    2. Is there more Javascript downloading?
    - Check both first- and third-party
    CASE FILES: FIRST INPUT DELAY (FID) @ksylor
    

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82. Resource Timing
    RESOURCE TIMING
    SUSPECT #2
    Timeline-based
    Diagnostic
    RUM and Synthetic
    @ksylor
    

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83. JS RESOURCES
    CASE FILES: FIRST INPUT DELAY (FID) @ksylor
    

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84. DIAGNOSING CHANGES TO FID
    1. Is there more Javascript execution, e.g. Long Tasks? YES!
    2. Is there more Javascript downloading? NO? I think?
    - Check both first- and third-party
    3. Did TBT or TTI get worse?
    CASE FILES: FIRST INPUT DELAY (FID) @ksylor
    

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85. Independent
    Primary - Core Web Vitals?
    Total Blocking Time
    TBT
    CASE FILES
    Synthetic
    @ksylor
    

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86. Timeline-based
    Primary
    Time to Interactive
    TTI
    CASE FILES
    Synthetic
    @ksylor
    

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87. TBT OR TTI?
    ✦ ~30 Synthetic tests were all inconclusive :(
    CASE FILES: FIRST INPUT DELAY (FID) @ksylor
    

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88. DIAGNOSING CHANGES TO FID
    1. Is there more Javascript execution, e.g. Long Tasks? YES!
    2. Is there more Javascript downloading? NO? I think?
    - Check both first- and third-party
    3. Did TBT or TTI get worse? NO
    4. Did user behavior change? IDK
    CASE FILES: FIRST INPUT DELAY (FID) @ksylor
    

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89. DIAGNOSING CHANGES TO FID
    1. Is there more Javascript execution, e.g. Long Tasks? YES!
    2. Is there more Javascript downloading? NO? I think?
    - Check both first- and third-party
    3. Did TBT or TTI get worse? NO
    4. Did user behavior change? IDK
    5. Can we get FID attribution please Yoav FFS
    CASE FILES: FIRST INPUT DELAY (FID) @ksylor
    

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90. Independent
    Primary - Core Web Vitals?
    Interaction to Next Paint
    INP
    CASE FILES
    RUM
    @ksylor
    ?????
    

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91. WHAT DID WE LEARN?
    IT ALL STARTS WITH
    MONITORING
    @ksylor
    

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92. WHAT DID WE LEARN?
    PERF DETECTING ISN'T
    MAGIC
    askastaffengineer.com
    @ksylor
    

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93. WHAT DID WE LEARN?
    IT DEPENDS
    askastaffengineer.com
    @ksylor
    

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94. !
    U
    Y
    k o
    T
    @ksylor
    hAn
    

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