My talk from brightonSEO 2018 covering various web performance strategies, this time mainly focussing on critical rendering path, various image optimisation strategies as well as how to handle custom web fonts.
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Translating experiences to performance metrics
User experience
▪ Is it happening?
› Did the navigation start successfully?
Has the server responded?
▪ Is it useful?
› Has enough content rendered for users
to engage with it?
▪ Is it usable?
› Can users interact with the page or is it
still busy loading?
▪ Is it smooth/delightful?
› Are the interactions smooth and
natural, free of lag and jank?
Corresponding metric
First Paint (FP)/First Contentful Paint (FCP)
First Meaningful Paint (FMP) -> Hero Element Timing
Time to Interactive (TTI)
Long tasks (technically the absence of those long tasks)
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Optimising and measuring for painting timings
#1 #2
First Paint (FP)
Time to First Paint – marks the point when the
browser starts to render something, the first bit of
content on the screen.
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Optimising and measuring for painting timings
#1 #2 #3 #4
First Paint (FP) First Contentful
Paint (FCP)
Time to First Paint – marks the point when the
browser starts to render something, the first bit of
content on the screen.
Time to First Contentful Paint – marks the point when
the browser renders the first bit of content from the
DOM, text, an image etc.
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Optimising and measuring for painting timings
#1 #2 #3 #4 #5 #6
First Paint (FP) First Contentful
Paint (FCP)
First Meaningful
Paint (FMP) / Hero!
Time to Interactive
(TTI)
Time to First Paint – marks the point when the
browser starts to render something, the first bit of
content on the screen.
First Meaningful Paint – the paint after which the
biggest above-the-fold layout change has happened
and your most important element is visible!
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Track paint timings with Google Analytics (in theory)
Get the tracking code snippets: http://pa.ag/2viHQSz
version 62 and up
You must ensure your
PerformanceObserver is
registered in the <head>
before any stylesheets, so it
runs before FP/FCP happens.
(a buffered flag TBD in v.2)
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This is how it looks like in Google Analytics
Behavior > events > pages: performance metrics [first-contentful-paint]
Source: Google Analytics
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CSSOM: the CSS Object Model
▪ The CSSOM is a “map” of the CSS styles
found on a web page.
▪ It’s much like the DOM (Document Object
Model), but for CSS rather than HTML.
▪ The CSSOM combined with the DOM is
used by browsers to display web pages.
body
font-size:16px;
h1
font-size:22px;
p
font-size:16px;
p
font-size:12px;
a
font-size:12px;
img
font-size:16px;
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Web browsers use the CSSOM to render a page
If this is external CSS, the browser
needs to wait for the download
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Google doesn’t make a single GET request for its CSS!
Because requesting external CSS is more expensive than in-lining everything.
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How to know which CSS is critically required?
“Critical” renders in multiple resolutions & builds a combined/compressed CRP CSS:
Critical & criticalCSS on GitHub: http://pa.ag/2wJTZAu & http://pa.ag/2wT1ST9
▪ Minimum: a snapshot of CSS rules to
render a default desktop resolution
(e.g. 1280x1024).
▪ Better: various snapshots for mobile
phones, pad/s & desktop/s – manually
that’d be a lot of work!
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If you want to play around first: Try criticalcss.com
Give it a try: http://pa.ag/2nVIwXB
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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<title>CRP loading demo</title>
<!-- critical CSS goes here -->
<style> h1 { colour: green; } </style>
<!-- use async preload // no IE, Edge & some other unimportant ones (http://caniuse.com/#search=preload) -->
<link rel="preload" href="non-critical.css" as="style" onload="this.rel='stylesheet'" />
<!--noscript for req. without JS -->
<noscript><link rel="stylesheet" href="non-critical.css"></noscript>
<!-- include polyfill for shitty browsers -->
<script>
*! loadCSS. [c]2017 Filament Group, Inc. MIT License */
(function(){ ... } ());
/*! loadCSS rel=preload polyfill. [c] 2017 Filament Group, Inc. MIT License */
(function(){ ... } ());
</script>
</head>
<body>
</body>
</html>
<!-- use async preload // no IE, Edge & some other unimportant ones
(http://caniuse.com/#search=preload) -->
<link rel="preload" href="non-critical.css" as="style" onload="this.rel='stylesheet'" />
<!-- critical CSS goes here -->
<style> h1 { colour: green; } </style>
<!-- use async preload // no IE, Edge & some other unimportant ones
(http://caniuse.com/#search=preload) -->
<link rel="preload" href="non-critical.css" as="style" onload="this.rel='stylesheet'" />
<!--noscript for req. without JS -->
<noscript><link rel="stylesheet" href="non-critical.css"></noscript>
*! loadCSS. [c]2017 Filament Group, Inc. MIT License */
(function(){ ... } ());
/*! loadCSS rel=preload polyfill. [c] 2017 Filament Group, Inc. MIT License */
(function(){ ... } ());
Putting it all together
Fit the HTML, CSS & JS that’s necessary for “Start Render” into that first 14 kB round trip!
Inline your critical CSS.
1
Loading non-critical CSS
async using rel=“preload“.
2
Apply the CSS once it has
finished loading via “onload“.
3
Fallback for non-JS requests.
4
Implement loadCSS script for
older browsers.
5
23. Let’s look at an implementation…
Is it worth the effort?
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Before & after: a fresh WordPress setup #1
HTTP, no HTTP/2, Twenty Seventeen theme (1x CSS, 8x JS, custom fonts), no caching
and no other performance optimizations
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Before & after: a fresh WordPress setup #2
HTTP, no HTTP/2, Twenty Seventeen theme (1x CSS, 8x JS, custom fonts), W3Total (CSS,
JS, HTML minify, caching, compression)
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Before & after: a fresh WordPress setup #3
HTTP, no HTTP/2, Twenty Seventeen theme (1x CSS, 8x JS, custom fonts), W3Total (CSS,
JS, HTML minify, caching, compression) + CRP CSS inlined
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Performance metrics comparison at a glance
Rendering starts significantly earlier; this allows for faster interaction with the site.
KPI / MEASUREMENT
Load Time
Time to First Byte (TTFB)
Start Render
Time to Interactive (TTI)
DEFAULT WP
1.357 sec
0.454 sec
1.000 sec
0.956 sec
BASICS (W3TOTAL)
0.791 sec
0.159 sec
0.600 sec
0.931 sec
FULLY OPTIMIZED
0.789 sec
0.157 sec
0.410 sec
0.563 sec
(+32%)
(+41%)
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TL;DR
Implement proper tracking, measure “First Meaningful Paint” (Hero Element delivery).
Audit, clean and (afterwards) split CSS into two parts: “initial view” and “below the fold”.
Use “critical” to generate and inline your critical path CSS.
Use rel=“preload“ and “loadCSS” to async load below the fold / site-wide CSS.
Off-load all overhead (JS, etc.) to stay within 14kB for faster, initial paint.
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62% of all web traffic is made up of images...
… and 51% of all URLs load more than 40 images per request.
Source: http://pa.ag/1SGDOEo
Average bytes per page by content type Image requests per page
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WebP: Google’s alternative to JPEG, PNG, and GIF
Lossy & lossless compression, transparency, metadata, colour profiles, animation, and
much smaller files (30% vs. JPEG, 80% vs. PNG) – but only in Chrome, Opera & Android
Everything about WebP: http://pa.ag/1EpFWeN / & WebP support: http://pa.ag/2FZK4XS
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You can still use WebP with on-the-fly replacement
Swap PNG and JPEG images per re-write (i.e., using .htaccess)
VS.
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There is way more: FLIF, BPG, JPEG-XR, etc.
If you’re “image-heavy”, go play with it!
Further reading: http://pa.ag/1S5OQmX
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SEMrush (blog) could save 80-90% of it’s image traffic
Better compression combined with modern image formats (i.e. WebP & JPEG-XR)
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Please DON’T use a 1.2 MB background image, Kelvin?
Seriously, this can be as small as 83 KB (PNG) or even 17 KB (WebP)!
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>70% of all websites use at least one non-standard font!
Result: 114 kB of additional data and on average 3 additional HTTP requests.
Source: http://pa.ag/1BRUnbe
Font transfer size & font requests Sites with custom fonts
Font transfer size (kB) Font requests
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Classic scenario: using external CSS
Easy to use with one big disadvantage: It’s render-blocking!
CSS (font) call to Google causes
the render to stop / block until
the download has been finished!
39. FOIT (flash of invisible text) or FOUT (flash of unstyled text)
can cause annoying flickering
Asynchronous?
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Fighting the flash of unstyled text/content
Make your fall-back font match the intended web font (letter spacing, heights, etc.)
Give it a try: https://pa.ag/2qgE8EH
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Fighting the flash of invisible text
New stuff to play around with: Various “font-display” strategies (no IE/Edge yet)
More: http://pa.ag/2eUwVob
‘font-display’ allows to display text while the font for it is still loading!
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Don‘t miss Monica Dinculescu‘s great talk titled
„Fontastic Web Performance“
Watch the full talk: https://pa.ag/2qf6hvK
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If you can only do one thing, I’d recommend doing this:
Go to your CSS file, look for @font-face
and add `font-display:optional` -
there hasn’t been a safer & easier gain
in #webperf in a long time!
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Come and say “hello” or apply via jobs.pa.ag. Looking forward to speaking to you!
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