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loading is a user journey
with many disparate expectations
you’ve probably heard to REDUCE DNS LOOKUPS, REDUCE ROUND-TRIP TIMES, MINIMIZE REDIRECTS, ELIMINATE UNN
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USERS LOOK FOR VISUAL FEEDBACK TO reassure
them everything is working as expected.
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first Interactive
consistently Interactive
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Time to Interactive
<5s
on an average mobile
device over 3G
*2s on repeat-load a:er Service Worker registered
GOAL
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19s
16s 420KB
JavaScript Startup Performance, Double-Click Mobile Speed Matters report & the HTTP Archive
The average web page on mobile in 2017
UNTIL INTERACTIVE FULLY LOADED JAVASCRIPT
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PERFORMANCE
JavaScript
Start-up Start-up
o o
V8 Runtime Call Stats
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1MB script
(250KB minified)
JS Parse Time On Mobile
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about:inspect in Chrome DevTools
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webpagetest.org/easy
Moto G4 + 3G
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Code-splitting
// Defines a “split-point” for a separate bundle
require.ensure([], () => {
const profile = require('./UserProfile', cb);
});
import('./UserProfile')
.then(loadRoute(cb))
.catch(errorLoading)
Webpack 2+
Webpack 1
Also see Splittable, Closure Compiler
or Browserify
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Do I need to split?
Try Code Coverage
in Chrome DevTools
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Tree-shaking
// app.js
import { a } from ‘./module.js’;
// module.js
export function a () {}
export function b () {}
❌
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Use babel-preset-env to only transpile code for browsers that need it
{
"presets": [
["env", {
"targets": {
"browsers": ["last 2 versions"]
}
}]
]
}
Only transpile what you need with
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workflow
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Minify _everything_
Babelified ES5 w/Uglify
ES2015+ with Babili
css-loader + minimize:true
Code-splitting
Dynamic import()
Route-based chunking
Tree-shaking
Webpack 2+ with Uglify
RollUp
DCE w/ Closure Compiler
Optimize “Vendor” libs
NODE_ENV=production
CommonsChunk + HashedModuleIdsPlugin()
Transpile less code
babel-preset-env + modules:false
Browserlist
useBuiltIns: true
Scope Hoisting:
Webpack 3
RollUp
Strip unused Lodash modules
lodash-webpack-plugin
babel-plugin-lodash
Fewer Moment.js locales
ContextReplacementPlugin()
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Plenty of lightweight options for mobile
Lower total cost on size + parse times from the get-go
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Byte savings @
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Brotli
Display Ads from Google now served
using Brotli compression!
40%
Data-savings up to
15% in aggregate over gzip
https://developers.googleblog.com/
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Brotli
Improved load time by 7% in India & 4% U.S
bit.ly/linkedin-brotli
Decreased the size of static assets by 20%
bit.ly/dropbox-brotli
17% improvement for largest JS bundles
bit.ly/certsimple-brotli
1.5 petabytes (million gigs) saved a day
bit.ly/playstore-brotli
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WebP
30% smaller than JPEG
25% smaller than PNG
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WebP
bit.ly/webp-format
Serving over 43B image requests a day
25-30%
savings for WebP on average (26% lossless)
Data Saver + Web Store
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WebP Conversion
XNConvert
Windows/Mac/Linux
Can convert in batch
Supports most formats
Alternatively:
imagemin
Pixelmator
ImageMagick
GIMP
Leptonica
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WebP Serving
![]()
Or use the Accept header + .htaccess to serve WebP if a browser supports it and it exists on disk.
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Service Workers
Inbox by Gmail
10%
improvement in Time-to-Interactive
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HTTP Caching Checklist
Use consistent URLs and minimize resource churn
Provide a validation token (ETag) to avoid transferring unchanged bytes
Identify resources that can be cached by intermediaries (like CDNs)
Determine the optimal cache lifetime of resources (max-age)
Consider a Service Worker for more control over your repeat visit caching
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5.
bit.ly/caching-checklist
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Let’s hack
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ResourceLoadPriorityVeryHigh
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ResourceLoadPriorityVeryHigh
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FLUENTinium
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Original
Everything is high priority
Preloaded JS
Preloaded CSS + fonts
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Original
Everything is high priority
JS + CSS is high priority
CSS + fonts are high prio
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..
Link: 1.js; rel="preload"; as="script"
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bit.ly/prpl-paFern
PRPL
bit.ly/prpl-paFern
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HTTP/2 with 3G
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HTTP/2 + preload with 3G
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Express + HTTP/2 Push Headers
const express = require('express'),
let app = express();
app
.use('/js', express.static('js'))
.get('/', function (req, res) {
res.set('Link', `
; rel=preload; as='style',
; rel=preload; as='script',
; rel=preload; as='script'`)
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Alternatively: Track cache content using cookies
if (supports_http2() && !http_cached('/app.js')) {
header('link:; rel=preload; as=script’);
setcookie('/app.js', 'is-cached', 0, '/');
}
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Alternatively: Track cache content using cookies
function http_cached($filename) {
if ('is-cached' === $_COOKIE[$filename]) {
return true;
} else {
return false;
}
}
Try CASPer
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Repeat visit with Service Worker
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Next: Differential Serving based on
browser compatibility?
HTTP/1 works better when resources are concatenated (bundled)
HTTP/2 works better when resources are more granular (unbundled)
Serve an unbundled build for server/browser combinations supporting
HTTP/2. Trigger delivery with or HTTP/2 Push
Serve a bundled build to minimize round-trips to get the app running on
server/browser combinations that don't support HTTP/2 Push
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Debugging: HTTP/2 Server Push in DevTools
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Debugging: HTTP/2 Server Push in DevTools
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HTTP/2 Server Push Rules Of Thumb
Push just enough resources to fill idle network time, and no more.
Push resources in evaluation-dependence order.
Consider using strategies to track the client-side cache.
Use the right cookies when pushing resources.
Use server push to fill the initial cwnd. Consider
preload links to reveal remaining critical resources.
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bit.ly/h2push
PUSH
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PRPL In-A-Box
Polymer App
Toolbox
PREACT
CLI
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With Service Workers
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With HTTP/2 Server Push
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babel-preset-env + per-browser bundles
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babel-preset-env + per-browser bundles
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Twitter Lite
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Interactive in <5s on 3G
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Can we get fast 3G numbers
across the board or regular
3G?
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PRPL
Push / Preload
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18% improvement
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36% improvement
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Render
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HTML Streaming
reduced TTFB by 30%
(200ms), increasing time
user’s spent in the app.
Nicolas Gallagher, Technical lead for Twitter Lite
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4x improvement to
render perf by using
requestIdleCallback() to
defer JS loading of
images.
Nicolas Gallagher, Technical lead for Twitter Lite
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Heavy image decode Lower image decode
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Adapt intelligently
H
E
I
G
H
T
Size appropriately
WIDTH
IMAGE DECODE
Compress carefully Take care with tools
Prioritize critical images
HIGH
LOW
Lazy-load the rest
Choose the right format
High-perf
Images
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Data Saver Mode introduced up to 70% savings
Next up: Save-Data client hint
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Precache
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This is a headline
Followed by a subhead
This is body copy and it goes a little like this and Lorem ipsum
dolor sit amet, consectetur adipiscing elit. This is body copy and
it goes a little like this and Lorem ipsum dolor sit amet,
consectetur adipiscing elit.
Application Shell
A skeleton representing the
user interface that can be
offline cached & instantly
rendered on repeat visits.
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Before Service Worker
After Service Worker
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Lazy-load
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Before code-splitting
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webpack-web.config.js
const plugins = [
// extract vendor and webpack's module manifest
new webpack.optimize.CommonsChunkPlugin({
names: [ 'vendor', 'manifest' ],
minChunks: Infinity
}),
// extract common modules from all the chunks (requires no
'name' property)
new webpack.optimize.CommonsChunkPlugin({
async: true,
children: true,
minChunks: 4
})
];
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After code-splitting
bit.ly/twitterlite-perf
bit.ly/twitter-casestudy
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https://meowni.ca/font-style-matcher/
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https://www.zachleat.com/web/comprehensive-webfonts/
“Comprehensive
Web Fonts”
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Link: ; rel=preload; as=font;
type='font/woff'
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Heaviest use of rel=preload is for Web Fonts
HTTPArchive
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Preloading Web Fonts = 50% (1.2s)
improvement in time-to-text-paint
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Control font performance
with font-display
auto: uses whatever font display strategy the user-agent uses
block: draws "invisible" text at first if the font is not loaded, but swaps
the font face in as soon as it loads
swap: draws text immediately with a fallback if the font face isn’t
loaded, but swaps the font face in as soon as it loads
fallback: font face is rendered with a fallback at first if it’s not loaded,
but the font is swapped as soon as it loads
optional: if the font face can’t be loaded quickly, just use the fallback
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bit.ly/font-subsetting
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https://fonts.googleapis.com/css?family=Inconsolata
https://fonts.googleapis.com/css?family=Inconsolata&text=Hello
Web Font Subsetting
~3KB
~880 bytes
Supported by Google Fonts
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/* Small subset, normal weight */
@font-face {
font-family: whatever;
src: url('reg-subset.woff')
format('woff');
unicode-range: U+0-A0;
font-weight: normal;
}
The browser can also handle subsetting!
https://jakearchibald.com/2014/minimising-font-downloads/
/* Large subset, normal weight */
@font-face {
font-family: whatever;
src: url('reg-extended.woff')
format('woff');
unicode-range: U+A0-FFFF;
font-weight: normal;
}
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CSS Font Loading API
https://developers.google.com/web/fundamentals/performance/optimizing-content-efficiency/webfont-optimization
const font = new FontFace("Awesome Font", "url(/fonts/awesome.woff2)", {
style: 'normal', unicodeRange: 'U+000-5FF', weight: '400'
});
// don't wait for the render tree, initiate an immediate fetch!
font.load().then(function() {
// apply the font (which may re-render text and cause a page reflow)
// after the font has finished downloading
document.fonts.add(font);
document.body.style.fontFamily = "Awesome Font, serif";
// OR... apply your own render strategy here...
});
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Web Font Loading Tips
Understand the anatomy of a web font and how browsers load
font-display: optional (i.e if you can’t do it fast, load a fallback)
Minimize font downloads by limiting range of characters you’re loading
Minimize FOIT by using
If you need more control try out the Font Loading API
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https://meowni.ca/posts/web-fonts/
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Streams API
bit.ly/streams-ftw
Progressive Loading: HTML
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in body
bit.ly/progressive-css
Progressive Loading: CSS
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in body
bit.ly/progressive-css
Progressive Loading: CSS
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medium.com/reloading
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