How to optimise frontend performance
This document is current until 6 December 2018
You should focus on frontend performance when developing your service’s website. This will improve the user experience of your service by making your website respond faster and work better on all devices.
Prioritise performance tasks
You can optimise your site’s frontend performance by prioritising tasks that will improve your site speed. Prioritise things you must do (high) over medium or low priority (nice to have) tasks.
|High||Position styles correctly||Set styles at the top and scripts at the bottom of a webpage|
|Set correct Headers||Set correct Cache-Control and ETag headers on assets for optimal caching|
|Medium||Look for empty image
||Avoid using empty image
|Minimise DNS lookups||Use fewer third-party domains to reduce the number of DNS lookups per page|
|Maximise parallelisation||Serve static assets from a different subdomain so browsers can download more assets in parallel|
|Investigate lazy loading||For pages with many images, only load images in the immediate browser viewport|
|Investigate the impact of loading multiple @font-face assets||Investigate @font-face assets when dealing with common issues like FOUT, FOIT and FOFT|
|Low||Set images and sprites correctly||Set images and sprites horizontally as it’s easier for browsers to parse|
|Reduce cookie size||Because every cookie is sent with each HTTP request, consider using a cookie-free domain for static assets|
|AJAX requests using JSON||Avoid adding too much data to a JSON Object because this causes performance errors with parsing|
|Investigate using WebSockets||Consider using WebSockets rather than XMLHttpRequest, because an HTTP request packet has 1,684 bytes of overhead, compared to 8 bytes for a WebSocket packet|
|Investigate using a service worker||Consider using a service worker to cache critical assets on users machines instead of transferring them over the network|
You can automate performance optimisation using tools such as:
You should integrate these tools into your Continuous Delivery (CD) and Continuous Integration (CI) workflow so they automatically run before deployment.
Consider automating common tasks like:
- CSS and JS Linting and optimisation
- CSS and JS Minification
- image optimisation
- sprite and icon generation
- SVG Image optimisation
Google PageSpeed can perform many of these tasks for you.
You can automate frontend performance testing during deployment using third-party services such as:
- Google PageSpeed Insights
You should set a performance budget for your website’s pages. Once you’ve set a performance budget, test to check your website’s pages stay within your budget. There are many tools available to do this, such as:
- PSI - PageSpeed Insights with reporting
You can find out more about improving your website’s frontend performance by reading:
- Setting a performance budget
- My performance audit workflow
- Front-end performance for web designers and front-end developers
- Improving web app performance with the Chrome DevTools Timeline and Profiles
- Google Web Fundamentals: Optimizing Content Efficiency
The Service Manual has more suggestions about how you can test frontend performance.