Front-End Performance Checklist 2021

This guide has been kindly supported by our friends at LogRocket, a service that combines frontend performance monitoring, session replay, and product analytics to help you build better customer experiences. LogRocket tracks key metrics, incl. DOM complete, time to first byte, first input delay, client CPU and memory usage. Get a free trial of LogRocket today.

Web performance is a tricky beast, isn’t it? How do we actually know where we stand in terms of performance, and what exactly our performance bottlenecks are? Is it expensive JavaScript, slow web font delivery, heavy images, or sluggish rendering? Have we optimized enough with tree-shaking, scope hoisting, code-splitting, and all the fancy loading patterns with intersection observer, progressive hydration, clients hints, HTTP/3, service workers and — oh my — edge workers? And, most importantly, where do we even start improving performance and how do we establish a performance culture long-term?

Back in the day, performance was often a mere afterthought. Often deferred till the very end of the project, it would boil down to minification, concatenation, asset optimization and potentially a few fine adjustments on the server’s config file. Looking back now, things seem to have changed quite significantly.

Performance isn’t just a technical concern: it affects everything from accessibility to usability to search engine optimization, and when baking it into the workflow, design decisions have to be informed by their performance implications. Performance has to be measured, monitored and refined continually, and the growing complexity of the web poses new challenges that make it hard to keep track of metrics, because data will vary significantly depending on the device, browser, protocol, network type and latency (CDNs, ISPs, caches, proxies, firewalls, load balancers and servers all play a role in performance).

So, if we created an overview of all the things we have to keep in mind when improving performance — from the very start of the project until the final release of the website — what would that look like? Below you’ll find a (hopefully unbiased and objective) front-end performance checklist for 2021 — an updated overview of the issues you might need to consider to ensure that your response times are fast, user interaction is smooth and your sites don’t drain user’s bandwidth.

Table Of Contents

(You can also just download the checklist PDF (166 KB) or download editable Apple Pages file (275 KB) or the .docx file (151 KB). Happy optimizing, everyone!)

Getting Ready: Planning And Metrics

Micro-optimizations are great for keeping performance on track, but it’s critical to have clearly defined targets in mind — measurable goals that would influence any decisions made throughout the process. There are a couple of different models, and the ones discussed below are quite opinionated — just make sure to set your own priorities early on.

    Introducing the slowest day of the week. Facebook has introduced 2G Tuesdays to increase visibility and sensitivity of slow connections. (Image source)

    Luckily, there are many great options that help you automate the collection of data and measure how your website performs over time according to these metrics. Keep in mind that a good performance picture covers a set of performance metrics, lab data and field data:

  • Synthetic testing tools collect lab data in a reproducible environment with predefined device and network settings (e.g. Lighthouse, Calibre, WebPageTest) and
  • Real User Monitoring (RUM) tools evaluate user interactions continuously and collect field data (e.g. SpeedCurve, New Relic — the tools provide synthetic testing, too).

The former is particularly useful during development as it will help you identify, isolate and fix performance issues while working on the product. The latter is useful for long-term maintenance as it will help you understand your performance bottlenecks as they are happening live — when users actually access the site.

By tapping into built-in RUM APIs such as Navigation Timing, Resource Timing, Paint Timing, Long Tasks, etc., synthetic testing tools and RUM together provide a complete picture of performance in your application. You could use Calibre, Treo, SpeedCurve, mPulse and Boomerang,, which all are great options for performance monitoring. Furthermore, with Server Timing header, you could even
monitor back-end and front-end performance all in one place.

Note: It’s always a safer bet to choose network-level throttlers, external to the browser, as, for example, DevTools has issues interacting with HTTP/2 push, due to the way it’s implemented (thanks, Yoav, Patrick!). For Mac OS, we can use Network Link Conditioner, for Windows Windows Traffic Shaper, for Linux , and for FreeBSD .

As it’s likely that you’ll be testing in Lighthouse, keep in mind that you can:

  1. Share the performance goals with your colleagues.
    Make sure that performance goals are familiar to every member of your team to avoid misunderstandings down the line. Every decision — be it design, marketing or anything in-between — has performance implications, and distributing responsibility and ownership across the entire team would streamline performance-focused decisions later on. Map design decisions against performance budget and the priorities defined early on.

Setting Realistic Goals

From Fast By Default: Modern loading best practices by Addy Osmani (Slide 19)

Defining The Environment

PRPL stands for Pushing critical resource, Rendering initial route, Pre-caching remaining routes and Lazy-loading remaining routes on demand.

An application shell is the minimal HTML, CSS, and JavaScript powering a user interface.

Assets Optimizations

The Responsive Image Breakpoints Generator automates images and markup generation.

Build Optimizations

Delivery Optimizations

Networking and HTTP/2

  1. Do we send all assets over a single HTTP/2 connection?
    One of the main advantages of HTTP/2 is that it allows us to send assets down the wire over a single connection. However, sometimes we might have done something wrong — e.g. have a CORS issue, or misconfigured the crossorigin attribute, so the browser would be forced to open a new connection.

Testing And Monitoring

Quick Wins

  1. Measure the real world experience and set appropriate goals. Aim to be at least 20% faster than your fastest competitor. Stay within Largest Contentful Paint < 2.5s, a First Input Delay < 100ms, Time to Interactive < 5s on slow 3G, for repeat visits, TTI < 2s. Optimize at least for First Contentful Paint and Time To Interactive.
  2. Trim, optimize, defer and lazy-load scripts. Invest in the config of your bundler to remove redundancies and check lightweight alternatives.
  3. Always self-host your static assets and always prefer to self-host third-party assets. Limit the impact of third-party scripts. Use facades, load widgets on interaction and beware of anti-flicker snippets.
  4. Be selective when choosing a framework. For single-page-applications, identify critical pages and serve them statically, or at least prerender them, and use progressive hydration on component-level and import modules on interaction.
  5. Client-side rendering alone isn’t a good choice for performance. Prerender if your pages don’t change much, and defer the booting of frameworks if you can. If possible, use streaming server-side rendering.
  6. Experiment with regrouping your CSS rules and test in-body CSS.
  7. Add resource hints to speed up delivery with faster dns-lookup, preconnect, prefetch, preload and prerender.
  8. Subset web fonts and load them asynchronously, and utilize font-display in CSS for fast first rendering.
  9. Check that HTTP cache headers and security headers are set properly.
  10. Enable Brotli compression on the server. (If that’s not possible, at least make sure that Gzip compression is enabled.)
  11. Enable TCP BBR congestion as long as your server is running on the Linux kernel version 4.9+.
  12. Enable OCSP stapling and IPv6 if possible. Always serve an OCSP stapled DV certificate.
  13. Enable HPACK compression for HTTP/2 and move to HTTP/3 if it’s available.
  14. Cache assets such as fonts, styles, JavaScript and images in a service worker cache.

Download The Checklist (PDF, Apple Pages)

Off We Go!

This content was originally published here.