[Confoo 26] Boosting Frontend Speed: Quick Wins for Backend Developers

by Paul Conroy

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Boosting Frontend Speed: Quick Wins for Backend Developers Paul Conroy / @conroyp

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Paul Conroy From Dublin, Ireland Started playing with the web 30+ years ago (Notepad, Frontpage & Geocities!) CTO at Square1 conroyp.com / @conroyp 👴 🌍 🇮🇪

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In the beginning ● First website in 1991 ● Plain text, links - simple! https://info.cern.ch/hypertext/WWW/TheProject.html

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In the beginning ● First website in 1991 ● Plain text, links - simple! ● Img tag! Scrolling marquees! ● Basic CSS

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In the beginning ● First website in 1991 ● Plain text, links - simple! ● Img tag! Scrolling marquees! ● Basic CSS

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● First website in 1991 ● Plain text, links - simple! ● Img tag! Scrolling marquees! ● Basic → More advanced CSS ● Interactivity (Java applets, Flash, Javascript) In the beginning

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Evolving ecosystem

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“The site is slow” - Your boss

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Improved metrics ● Stand-alone metrics ● Objectively quantify performance ● Core Web Vitals - how does the site interaction “feel”? ● CLS, LCP, INP are key metrics

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Improved metrics ● Stand-alone metrics ● Objectively quantify performance ● Core Web Vitals - how does the site interaction “feel”? ● CLS, LCP, INP are key metrics

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Improved metrics ● CLS - how much do things jump around on a page? ● LCP - how long does to draw the biggest thing on the page? ● INP - how long between clicking and seeing something happen?

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● Uses same data Google uses in rankings ● Based on real-world Chrome user data ● More than page speed! Sitemaps, crawl errors, inbound links https://search.google.com/search-console/ Search Console

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https://pagespeed.web.dev/ ● Aggregate score 1 - 100 Page Speed Insights

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https://pagespeed.web.dev/ ● Aggregate score 1 - 100 ● Separate score for individual CWV metrics ● Detailed information on reasons behind failing metrics Page Speed Insights

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● Aggregate score 1 - 100 ● Separate score for individual CWV metrics ● Detailed information on reasons behind failing metrics ● Lab and “real world” data https://pagespeed.web.dev/ Page Speed Insights

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● Runs locally in Chrome ● Same as Page Speed Insights ● Local conditions impact results (cookies, cache, etc) ● Potential for significant variations run-to-run Lighthouse

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Lighthouse-PHP https://spatie.be/docs/lighthouse-php/v2/introduction

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Lighthouse

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Lighthouse “?noads=1” url toggle to get a “raw” result 💡

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How exactly are we meant to improve a slow website?

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Option 1

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Option 1

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Option 1 ● Send the team snowboarding for 6 months ● Browsers and hardware will improve in that time ● Problem solved

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Option 1 ● Send the team snowboarding for 6 months ● Browsers and hardware will improve in that time ● Problem solved ● Might need an Option 2…

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Images Ads Videos Fonts

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Get rid of them all!

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Improve page speed score?

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✅ Improve page speed score

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😎 🍹 🏖 ✅ Improve page speed score

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Images

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● Images are ~60% of average page load ● More responsive sites → more image sizes being generated ● Tricky to manage when handling user- generated content! IMAGES JS HTML CSS, FONTS

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Image sizing ● Stops the image from appearing too large

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Image sizing ● Stops the image from appearing too large ● Still requires a large image download

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● Stops the image from appearing too large ● File size is appropriate to the image being shown Image sizing ● Stops the image from appearing too large ● Still requires a large image download

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Choosing the right size ● Detecting the browser size based on User Agent / headers ● Generate different markup based on browser size

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Choosing the right size ● Detecting the browser size based on User Agent / headers ● Generate different markup based on browser size ● Impact on URL-based caching ● How does it handle device resizing after load?

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Responsive images ● Multiple sizes specified in markup ● Browser picks best one to load ● A single URL cache works fine! ● Resizing of the browser?  Right image size loaded

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Smaller images? ● Jpeg was the standard ● Webp: 25 - 30% smaller than jpeg  Minimal quality drop ● Avif: good for high end images  Can be 10-15% smaller than Webp ● JPEG XL is 10-15% smaller than AVIF

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https://caniuse.com/?search=webp

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https://caniuse.com/?search=avif

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https://caniuse.com/?search=jpegxl

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● Content type hints ● Browser will serve the first image it can ● Fallback to safer defaults

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● Important to “reserve” space! ● Tell the browser how much space we’ll need ● Different sizes on different devices? Define heights in media queries for each breakpoint

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Impact on page speed

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Impact on page speed

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Lazy loading ● Only load assets when they are needed ● Potentially save a lot of bandwidth ● Load images just before they scroll into view

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Lazy loading ● Only load assets when they are needed ● Potentially save a lot of bandwidth ● Load images just before they scroll into view

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THEN ● Load polyfill library ● When image top is near viewport, swap data-src and src ● Scroll listener thrashed rendering thread! Polyfills

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Polyfills THEN ● Load polyfill library ● When image top is near viewport, swap data-src and src ● Scroll listener thrashed rendering thread!

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NOW ● loading=”lazy” 🔥🔥🔥 ● Works on iframes too! Polyfills

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https://caniuse.com/?search=lazy

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Lazyload everything?

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● Tempting to loading=”lazy” all images ● Actively harmful if applied to images above the fold ● Browser is trying to optimise loading - help it out! ● Use fetchpriority=”high” as a signal Lazyload everything?

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● Specify relative priority with fetchpriority ● loading=”eager” is more defensive (templates defaulting to lazy)

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Lazy loading background images ● Background images usually large, full width, lots of detail ● CSS background-image ● Limited polyfill support for background images

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Lazy loading background images ● Background images usually large, full width, lots of detail ● CSS background-image ● Limited polyfill support for background images ● IntersectionObserver replaces window.scroll

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IntersectionObserver

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IntersectionObserver

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IntersectionObserver

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IntersectionObserver

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IntersectionObserver

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Images Ads Videos Fonts

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Display ads ● We have no way of knowing if an ad will show or not at render time ● Page content “jumping” - very bad CLS ● Very annoying for users! ● Accidental clicks, maybe good for site owners?

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Two-click penalty ● Google adds overlay on ads ● Asks user to click again ● Google don’t tell you this is happening! ● It doesn’t show to every user ● How do we know when it’s happening?

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Two-click penalty 📉 📉 💸 ● Google adds overlay on ads ● Asks user to click again ● Google don’t tell you this is happening! ● It doesn’t show to every user ● How do we know when it’s happening?

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Avoiding the Two-click Penalty ● Label ads clearly ● Consistent spacing from content ● Avoid areas near menus, buttons ● Prevention is better than cure!

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Reducing CLS ● Min-height on the ad container, matching expected ad unit size ● Same technique as with images

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● Min-height on the ad container, matching expected ad unit size ● Same technique as with images ● Ad loads in, taking the reserved space, no CLS or “Jumping” Reducing CLS

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● Min-height on the ad container, matching expected ad unit size ● Same technique as with images ● Ad loads in, taking the reserved space, no CLS or “Jumping” Reducing CLS

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Variable ad sizes? ● Responsive ad sizes (single request) ● Google might give us more than we ask for! ● How do we reserve the right space for each ad possibility?

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Variable ad sizes? ● Responsive ad sizes (single request) ● Google might give us more than we ask for! ● How do we reserve the right space for each ad possibility?

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Callbacks ● slotRenderEnded callback ● When an ad loads, check what div it was in ● If ad is smaller than our min height, remove the min height

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Callbacks ● Set min height to that of most popular ad unit ● “Bakes in” some CLS, but should be net positive

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Results ● Users experiencing bad CLS fell by 64% overnight ● Regular review of most popular slots is critical

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Images Ads Videos Fonts

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User Engagement & SEO benefits! ● Popular way to share content ● Strong user engagement ● Helps with SEO ● Very expensive payloads!

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Performance impact?

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Performance impact?

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Web components Lite Youtube Embed by Paul Irish This component: ● Loads a placeholder image ● On hover warms the connection to youtube ● On click, swaps the image for the player code

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Web components Lite Youtube Embed by Paul Irish This component: ● Loads a placeholder image ● On hover warms the connection to youtube ● On click, swaps the image for the player code

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Performance impact?

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Performance impact?

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Performance impact? 🐢 🐰 ⚡⚡⚡

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SEO impact ● Google is good at recognising embedded videos! ● Doesn’t recognise components so well ● Use structured data alongside custom components ● VideoObject schema https://schema.org/VideoObject

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Images Ads Videos Fonts

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Web fonts ● Brand “personality”, can brighten up dull sites ● Customisation has a cost! ○ Network requests ○ Render-blocking ○ Layout shifts ● Feel like design, behaves like infrastructure

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CLS from font loading

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CLS from font loading

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CLS from font loading

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CLS from font loading

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CLS from font loading

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● Variable fonts ● Stores master outlines ● Specifying the range, not each value

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I am loading: Best loading strategy: 1 font weight Single-weight font 2-3 font weights Depends - usually variable 4+ font weights Variable weight When should I use a variable font?

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What is a font?

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Font subsetting ● Remove all glyphs we’re not using ● Significant reduction in file sizes ● Support in modern tooling (vite, next.js, Google Fonts)

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Font subsetting pyftsubset Inter.var.ttf \ --unicodes=U+0020-007F \ --flavor=woff2 \ --layout-features='*'

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Font subsetting pyftsubset Inter.var.ttf \ --text-file=site-content.txt \ --flavor=woff2 \ --layout-features='*'

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470 kB → 83 kB

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470 kB → 83 kB

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470 kB → 83 kB

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470 kB → 83 kB 350 kB → 72 kB

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● Faster fonts, unblock rendering thread sooner ● LCP is probably an image, but is in a queue ● Is this data telling me what I think it is?

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Incidents By Driver

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● When dealing with image issues, have we other blocking resources? ● Improving font loading unblocks threads - faster image loading! ● Holistic view - top line metrics rarely influenced by solely one thing

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Images Ads Videos Fonts

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What’s next?

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RICK VISCOMI ● Web Perf DevRel at Chrome ● Optimisations for CWV ● CrUX report data https://bsky.app/profile/rviscomi.dev WEB.DEV ● Core Web Vitals ● Modern browser technique guidance ● Case studies, real world metrics https://web.dev/

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Images Control what loads and when ● Smaller file sizes: AVIF → Webp → Jpeg ● Lazy loading ● fetchpriority for hero images Ads Don’t let layouts move ● Reserve space ● Avoid the 2-click penalty! ● Labelling and positioning are key Videos Load complexity only on interaction ● Use facades inj place of heavy embeds ● SEO impact! Structured data objects Fonts Reduce Weight. Prevent Shift ● Variable fonts ● Subset intentionally ● Use size-adjust for fallbacks

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Thank you! 🌍 🌐 📧 Conroyp.com @conroyp [email protected]