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The Critical Request
Ben Schwarz
@benschwarz
!
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calibreapp.com
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Performance status quo
Follow the rules you know and love
• Serve less requests
• Use compression everywhere possible (Brotli, GZip)
• Bundle and minimise scripts (UglifyJS)
• Use asset hashing (app-0ff97b5.js), with “forever” cache headers
• Use CDNs (Fastly, Netlify, Cloudflare, Cloudfront)
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Performance through the eyes
of your users.
1.
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Users lose focus after 1 second.
Abandonment rises dramatically after 10 seconds.
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No content
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User navigates
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User navigates
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User navigates
First Paint
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User navigates
First Paint
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User navigates
First Paint
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User navigates
First Paint First Contentful Paint
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User navigates
First Paint First Contentful Paint
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User navigates
First Paint First Contentful Paint
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User navigates
First Paint First Contentful Paint
First Meaningful Paint
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User navigates
First Paint First Contentful Paint
First Meaningful Paint
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User navigates
First Paint First Contentful Paint
First Meaningful Paint
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Load + Render Timeline
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Load + Render Timeline
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onLoad
Load + Render Timeline
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onLoad
First Meaningful Paint
Load + Render Timeline
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onLoad
First Meaningful Paint
Load + Render Timeline
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Focus on these requests
onLoad
First Meaningful Paint
Load + Render Timeline
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onLoad
First Meaningful Paint
First Meaningful Paint
Text & Lead content has been rendered
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← Critical requests
A critical request is one that contains
an asset that is essential to the content
within the users viewport
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Often, these are:
1. CSS for the elements
on the page
2. Fonts
3. Logo
4. A lead image
← Critical requests
A critical request is one that contains
an asset that is essential to the content
within the users viewport
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How do browsers decide which
resources should be fetched?
2.
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Resource queuing
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My website
…
Resource queuing
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My website
…
Resource queuing
Queue
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My website
…
app.css
Resource queuing
Queue
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My website
…
app.css
app.js
Resource queuing
Queue
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My website
…
app.css
app.js
font.woff
Resource queuing
Queue
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Inspecting request priority
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Inspecting request priority
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No content
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No content
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app.css
app.js
font.woff
Assets are prioritised by type
Queue
Highest
High
Lowest
… and how they are referenced
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app.css
app.js
font.woff
Assets are prioritised by type
Queue
Highest
High
Lowest
• HTML (Highest)
… and how they are referenced
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app.css
app.js
font.woff
Assets are prioritised by type
Queue
Highest
High
Lowest
• HTML (Highest)
• Styles (Highest)
… and how they are referenced
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app.css
app.js
font.woff
Assets are prioritised by type
Queue
Highest
High
Lowest
• HTML (Highest)
• Styles (Highest)
• Images (Low or Medium)
… and how they are referenced
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app.css
app.js
font.woff
Assets are prioritised by type
Queue
Highest
High
Lowest
• HTML (Highest)
• Styles (Highest)
• Images (Low or Medium)
• XHR/Fetch (High)
… and how they are referenced
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app.css
app.js
font.woff
Assets are prioritised by type
Queue
Highest
High
Lowest
• HTML (Highest)
• Styles (Highest)
• Images (Low or Medium)
• XHR/Fetch (High)
• Fonts (Highest)
… and how they are referenced
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app.css
app.js
font.woff
Assets are prioritised by type
Queue
Highest
High
Lowest
• HTML (Highest)
• Styles (Highest)
• Images (Low or Medium)
• XHR/Fetch (High)
• Fonts (Highest)
• Scripts (Low, Medium or High!)
… and how they are referenced
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Javascript request priorities
For an in depth guide, see https://www.html5rocks.com/en/tutorials/speed/script-loading/ by @jaffathecake
High (when placed before ![]()
)
Otherwise, Medium
Low Priority
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How fonts are fetched
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How fonts are fetched
1. The stylesheet targets an element with a font declaration
body { font-family: Calibre; }
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How fonts are fetched
1. The stylesheet targets an element with a font declaration
body { font-family: Calibre; }
2. The stylesheet has a corresponding @font-face declaration
@font-face {
font-family: "Calibre";
src: url("/fonts/calibre.woff2") format("woff2"),
url("/fonts/calibre.woff") format("woff");
}
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How fonts are fetched
1. The stylesheet targets an element with a font declaration
body { font-family: Calibre; }
3. There is text to render
2. The stylesheet has a corresponding @font-face declaration
@font-face {
font-family: "Calibre";
src: url("/fonts/calibre.woff2") format("woff2"),
url("/fonts/calibre.woff") format("woff");
}
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Auditing and improving performance.
3.
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Auditing
atlassian.com
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Test using poor conditions
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Test using poor conditions
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Create a performance trace
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Create a performance trace
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No content
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No content
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Preload
Critical, essential resources.
Required for fonts!
HTML
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Preload
Critical, essential resources.
Required for fonts!
HTML
Link: ; rel=preload; as=font; crossorigin
As a HTTP Header
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Local overrides
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Local overrides
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Use preload for critical resources
Every browser that supports preload
also supports WOFF. Preload those!
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HTML
CSS
Font Weight 1
Font Weight 2
Font Weight 3
Network timeline
Text gets rendered here
Before preload
5-7 seconds until text is visible
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HTML
CSS
Font Weight 1
Font Weight 2
Font Weight 3
Network timeline
After preload
2-3 seconds until text is visible
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HTML
CSS
Font Weight 1
Font Weight 2
Font Weight 3
Network timeline
After preload
2-3 seconds until text is visible
Text gets rendered here
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@font-face {
font-family: LLCircularWeb;
src: url(/lineto-circular-black-c.woff) format("woff");
font-weight: 800;
font-style: normal;
font-display: swap;
}
Add font-display: swap
“
Display the text until the web font has loaded, then swap it out.
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HTML
Network timeline
Before font-display
2-3 seconds until text is visible
HTML
CSS
Font Weight 1
Font Weight 2
Font Weight 3
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HTML
Network timeline
Before font-display
2-3 seconds until text is visible
HTML
CSS
Font Weight 1
Font Weight 2
Font Weight 3
Text gets rendered here
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HTML
CSS
Font Weight 1
Font Weight 2
Font Weight 3
Network timeline
After font-display
Text is always visible
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HTML
CSS
Font Weight 1
Font Weight 2
Font Weight 3
Network timeline
After font-display
Text is always visible
Text gets rendered here
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Font style matcher
https://meowni.ca/font-style-matcher/
Monica Dinculescu
@notwaldorf
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Font style matcher
https://meowni.ca/font-style-matcher/
Monica Dinculescu
@notwaldorf
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Audit
improvement
notes
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Audit
improvement
notes
1. Use to
preload essential fonts.
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Audit
improvement
notes
1. Use to
preload essential fonts.
2. Use font-display: swap; to ensure
text is always visible.
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Audit
improvement
notes
1. Use to
preload essential fonts.
2. Use font-display: swap; to ensure
text is always visible.
3. Have font fallbacks that look similar to
avoid changes when webfonts load.
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Audit
improvement
notes
1. Use to
preload essential fonts.
2. Use font-display: swap; to ensure
text is always visible.
3. Have font fallbacks that look similar to
avoid changes when webfonts load.
4. Use woff2, woff where possible.
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Priority Hints
github.com/WICG/priority-hints
await fetch(‘api/endpoint.json’, {importance: ‘low’})
Experimental
await fetch(‘api/endpoint.json’, {importance: ‘low’})
Experimental
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Audit
checklist
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Audit
checklist
1. Test under poor conditions to highlight
performance issues.
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Audit
checklist
1. Test under poor conditions to highlight
performance issues.
2. Use the performance panel to explore
the relationship between render, asset
fetching, paints and JavaScript execution.
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Audit
checklist
1. Test under poor conditions to highlight
performance issues.
2. Use the performance panel to explore
the relationship between render, asset
fetching, paints and JavaScript execution.
3. Ensure critical requests are prioritised.
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Audit
checklist
1. Test under poor conditions to highlight
performance issues.
2. Use the performance panel to explore
the relationship between render, asset
fetching, paints and JavaScript execution.
3. Ensure critical requests are prioritised.
4. Iterate from good to great.
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Audit
checklist
1. Test under poor conditions to highlight
performance issues.
2. Use the performance panel to explore
the relationship between render, asset
fetching, paints and JavaScript execution.
3. Ensure critical requests are prioritised.
4. Iterate from good to great.
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Becoming a successful performance
advocate at your company.
4.
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Project timeline
Performance usually happens here
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Project timeline
“Hey, is the website fast?”
“ahhhh, um…”
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Project timeline
“Let’s make this really great!”
When performance should happen
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It ain't done until it's fast
It ain't done until it's accessible
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…
Can we schedule some more
performance work? I have no
idea how long it’ll take but we’ll
hope for the best!
Your manager
You, without a plan
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Performance workbook
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Performance workbook
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Performance workbook
Estimated gain Achieved gain Difficulty Estimated cost
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Performance workbook
Estimated gain Achieved gain Difficulty Estimated cost
Preload fonts 3 — 5 seconds FMP 3 seconds to FMP Easy Low
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Performance workbook
Estimated gain Achieved gain Difficulty Estimated cost
Preload fonts 3 — 5 seconds FMP 3 seconds to FMP Easy Low
Font-display: swap Instant text rendering Instant text rendering Easy Low
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Performance workbook
Estimated gain Achieved gain Difficulty Estimated cost
Preload fonts 3 — 5 seconds FMP 3 seconds to FMP Easy Low
Font-display: swap Instant text rendering Instant text rendering Easy Low
Remove all webfonts Less download? ?
Political hurdles with
branding team
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Performance workbook
Estimated gain Achieved gain Difficulty Estimated cost
Preload fonts 3 — 5 seconds FMP 3 seconds to FMP Easy Low
Font-display: swap Instant text rendering Instant text rendering Easy Low
Remove all webfonts Less download? ?
Political hurdles with
branding team
Rewrite app in React ? ? Hard Very expensive
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Performance workbook
Estimated gain Achieved gain Difficulty Estimated cost
Preload fonts 3 — 5 seconds FMP 3 seconds to FMP Easy Low
Font-display: swap Instant text rendering Instant text rendering Easy Low
Remove all webfonts Less download? ?
Political hurdles with
branding team
Rewrite app in React ? ? Hard Very expensive
Remove ads
10 seconds time to interactive
improvement on mobile devices
? Medium All our revenue
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Performance workbook
Estimated gain Achieved gain Difficulty Estimated cost
Preload fonts 3 — 5 seconds FMP 3 seconds to FMP Easy Low
Font-display: swap Instant text rendering Instant text rendering Easy Low
Remove all webfonts Less download? ?
Political hurdles with
branding team
Rewrite app in React ? ? Hard Very expensive
Remove ads
10 seconds time to interactive
improvement on mobile devices
? Medium All our revenue
Delete all Javascript No JS, no problems ? Easy
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Sure
Can we schedule some more
performance work? I’ve
summarised some research
and ideas in this spreadsheet…
I can run you through the
details over a coffee?
Your manager
You, an intellectual
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The
performance
advocate’s
manifesto
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The
performance
advocate’s
manifesto
1. Performance is a baseline requirement.
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The
performance
advocate’s
manifesto
1. Performance is a baseline requirement.
2. Every addition has a cost, therefore it
should have a value.
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The
performance
advocate’s
manifesto
1. Performance is a baseline requirement.
2. Every addition has a cost, therefore it
should have a value.
3. Performance testing should be automated.
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The
performance
advocate’s
manifesto
1. Performance is a baseline requirement.
2. Every addition has a cost, therefore it
should have a value.
3. Performance testing should be automated.
4. Draw conclusions and act on observed
performance data.
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The
performance
advocate’s
manifesto
1. Performance is a baseline requirement.
2. Every addition has a cost, therefore it
should have a value.
3. Performance testing should be automated.
4. Draw conclusions and act on observed
performance data.
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calibreapp.com
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@benschwarz
Cheers
calibreapp.com