A (Fairly) Complete Guide to Performance Budgets [SmashingConf SF 2023]

It's easier to make a fast website than it is to keep a website fast. If you've invested countless hours in speeding up your pages, but you're not using performance budgets to prevent regressions, you could be at risk of wasting all your efforts.

In this talk delivered at Smashing Conference SF in 2023, , we'll cover how to:

• Understand the difference between performance budgets and performance goals
• Identify which metrics to track
• Validate your metrics to make sure they're measuring what you think they are – and to see how they correlate with your user experience and business metrics
• Determine your budget thresholds
• Get buy-in from different stakeholders in your organization
• Integrate with your CI/CD process
• Maintain your budgets so you stay fast!

Transcript

1. A (Fairly) Complete Guide to Performance Budgets @tameverts #SmashingConf
2. @tameverts webperf.social/@tammy
3. @tameverts
4. speedcurve.com/benchmarks
5. speedcurve.com/newsletter/
7. Improved average load time from 6s à 1.2s 7-12% increase in conversion rate + 25% increase in PVs Average load time degraded to 5s User feedback: “I will not come back to this site again.” Re-focused on performance 0.4% increase in conversion rate 2010 2011 2009 @tameverts
8. 1. Constant feature development 2. Badly implemented third-parties 3. Waited too long to tackle problems 4. Relied on performance sprints 5. Stopped doing front-end performance measurement 6. No way to track regressions @tameverts
11. timkadlec.com/2013/01/setting-a-performance-budget/
12. csswizardry.com/2020/01/performance-budgets-pragmatically/
13. speedcurve.com/customers/tag/performance-budgets/
14. “Fighting regressions took priority over optimizations. The reason we decided this was because in the past, when we had performance efforts, engineers who were working on optimizations couldn’t really see progress in our performance metrics, because there were so many regressions happening at the same time.” Michelle Vu, Pinterest perfnow.nl/2018#michelle
16. What is a performance budget? Which metrics should I focus on? What should my budget thresholds be? How can I stay on top of my budgets? @tameverts
17. What is a performance budget?
18. Thresholds YOU create for metrics that are meaningful for YOUR site Time-based • Start Render, Largest Contentful Paint Quantity-based • Page size, image weight, Long Tasks Score-based • Cumulative Layout Shift, Lighthouse scores @tameverts
19. Monitoring tools Synthetic (lab) Mimics defined network & browser conditions No installation required Limited URLs Limited test locations Compare any sites Detailed analysis & visuals Real user monitoring (field) Real network & browser conditions Requires JavaScript installation Large sample size (up to 100%) Geographic spread Only measure your own site Correlation with other metrics (e.g., bounce rate) @tameverts
21. A good performance budget shows you… q What your budget is q When you go out of bounds q How long you’re out of bounds q When you’re back within budget @tameverts
22. Budgets can be passive (e.g. charts) Get alerts so you can investigate Even break the build
23. budget exceeded fixed @tameverts oops
24. Which metrics should I focus on?
26. TTFB DNS FCP FMP LCP CLS FID TBT TTI INP OMG WTF
28. 50+ synthetic metrics 30+ RUM metrics 25 additional metrics for EACH third party ∞ custom metrics
29. MVB
30. MVB Minimum Viable Budget
31. Is the page loading? Can I use it? How does it feel?
32. ❑ Tracks the most important content ❑ Is accessible out of the box ❑ Is available in synthetic and RUM ❑ Can be correlated to UX and business metrics ❑ Has broad browser support The ideal UX metric… @tameverts
33. Is the page loading?
34. Backend time The time from the start of the initial navigation until the first byte is received by the browser (AKA Time to First Byte) Synthetic & RUM
36. ❑ Tracks the most important content ❑ Is accessible out of the box ❑ Has broad browser support ❑ Is available in synthetic and RUM ❑ Can be correlated to UX and business metrics √ √ √ Backend time √
37. Start Render The time from the start of navigation until the first non-white content is painted Synthetic & RUM
39. As Start Render improves, so does bounce rate (typically) @tameverts
40. So does conversion rate (again, typically) @tameverts
41. ❑ Tracks the most important content ❑ Is accessible out of the box ❑ Has broad browser support ❑ Is available in synthetic and RUM ❑ Can be correlated to UX and business metrics √ √ √ Start Render √ √ @tameverts
42. Can I use the page?
43. 43 When do users start interacting with a page? @tameverts
44. Largest Contentful Paint (LCP) Amount of time it takes for the largest visual element (image or video) to render Synthetic & RUM (Chromium browsers only)
46. Largest Contentful Paint (often) correlates to bounce rate @tameverts
47. ❑ Tracks the most important content ❑ Is accessible out of the box ❑ Has broad browser support ❑ Is available in synthetic and RUM ❑ Can be correlated to UX and business metrics √ √ Largest Contentful Paint √ √ @tameverts
48. Last Painted Hero (LPH) When the last piece of critical content (hero image, first H1) is painted in the browser. Synthetic
50. ❑ Tracks the most important content ❑ Is accessible out of the box ❑ Has broad browser support ❑ Is available in synthetic and RUM ❑ Can be correlated to UX and business metrics √ √ Last Painted Hero √ @tameverts
51. How does the page feel?
52. Cumulative Layout Shift (CLS) Aggregate score that reflects how much page elements shift during rendering Synthetic & RUM (Chromium browsers only)
54. Bounce rate gets worse as CLS degrades Bounce rate improves as CLS degrades Bounce rate stays the same as CLS degrades speedcurve.com/blog/google-cumulative-layout-shift/
55. ❑ Tracks the most important content ❑ Is accessible out of the box ❑ Has broad browser support ❑ Is available in synthetic and RUM ❑ Can be correlated to UX and business metrics √ √ Cumulative Layout Shift @tameverts √
56. Long Tasks Any JavaScript function that takes >50ms to execute Long Tasks don’t always block page rendering, but they can cause the page to feel janky Synthetic & RUM
58. speedcurve.com/blog/first-input-delay-google-core-web-vitals/ Long Tasks Time often correlates to conversions @tameverts
59. 59 Long Tasks Time can expose issues with third parties @tameverts
60. ❑ Tracks the most important content ❑ Is accessible out of the box ❑ Has broad browser support ❑ Is available in synthetic and RUM ❑ Can be correlated to UX and business metrics √ Long Tasks @tameverts √ √ √
61. Interaction to Next Paint (INP) Measures a page's responsiveness to individual user interactions Observes latency and reports a single value that all (or nearly all) interactions are below RUM
62. speedcurve.com/blog/interaction-to-next-paint-core-web-vitals/
63. ❑ Tracks the most important content ❑ Is accessible out of the box ❑ Has broad browser support ❑ Is available in synthetic and RUM ❑ Can be correlated to UX and business metrics √ Interaction to Next Paint @tameverts √ √
64. Meaningful content Usable out of the box Broad browser support Synthetic RUM Correlates to business/UX Time to First Byte ★ ★ ★ ★ ☆ Start Render ☆ ★ ★ ★ ★ ★ Largest Contentful Paint ☆ ★ ★ ★ ★ Last Painted Hero ☆ ★ ★ ★ Cumulative Layout Shift ☆ ☆ ★ ★ Long Tasks ★ ★ ★ ★ ☆ Interaction to Next Paint ★ ★ ★ ★ @tameverts
65. Page weight & complexity
67. @tameverts
68. YMMV
69. What should my budget thresholds be?
70. performance budgets ≠ performance goals
71. Goals are aspirational How fast do I want to be eventually? Budgets are pragmatic How can I keep my site from getting slower while I work toward my goals? @tameverts
74. X
75. demoralizing not actionable ignorable
76. As you optimize your pages (and hopefully get faster!) update your budgets
77. How do I manage my budgets?
78. Should I set up budgets in RUM, synthetic, or both?
79. Scenario 1 Create budgets in RUM, and drill down via synthetic
80. @tameverts
81. Scenario 2 Create budgets in synthetic, and integrate with CI/CD
82. @tameverts
83. Should I create performance budgets for every metric I track?
87. Do I need performance budgets for all my pages?
88. 1. Product 2. Category 3. Home 4. Cart 5. Search @tameverts
89. Who should be responsible for managing budgets?
91. Everyone* who touches a page should understand the performance impact of their choices *Yes, this includes marketing people
92. For example… If your marketing team is responsible for adding and maintaining third-party tags, they should: q Have a basic understanding of the metrics (such as Long Tasks Time) q Collaborate on setting the budget q Receive alerts when the budget is violated q Participate (or at least have visibility) in identifying and fixing the issue
94. 1. Start small (even a single metric will do!) 2. Visually validate your metrics (filmstrips, videos) 3. Validate your metrics some more (UX, business) 4. Get buy-in from different stakeholders 5. Focus on the pages that matter most 6. Revisit your budgets regularly (2-4 weeks) 7. Remember that metrics are always evolving 8. Never stop measuring
95. Thank you! @tameverts webperf.social/@tammy
96. A Complete Guide to Performance Budgets speedcurve.com/blog/performance-budgets/ Setting a Performance Budget timkadlec.com/2013/01/setting-a-performance-budget/ Performance Budgets, Pragmatically csswizardry.com/2020/01/performance-budgets-pragmatically/ Web Vitals web.dev/vitals/ Farewell FID… hello Interaction to Next Paint speedcurve.com/blog/interaction-to-next-paint-core-web-vitals/ Cumulative Layout Shift: What it measures (and what it doesn’t) speedcurve.com/blog/google-cumulative-layout-shift/