Table of contents

The GDS Way and its content is intended for internal use by the GDS community.

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.

For example:

Priority Example Action
High Position styles correctly Set styles at the top and scripts at the bottom of a webpage
Minimise HTTP requests Minimise tiling icons, CSS and JavaScript files to reduce size and loading time
Compress static resources Use minification and Gzip to compress CSS and JavaScript code
Set correct Headers Set correct Cache-Control and ETag headers on assets for optimal caching
Medium Look for empty image src attributes Avoid using empty image src attributes as some browsers always send requests to them, resulting in additional traffic
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

Automate optimisation

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.

Automate testing

You can automate frontend performance testing during deployment using third-party services such as:

  • WebPagetest
  • Google PageSpeed Insights
  • SpeedCurve

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:

  • grunt-prefbudget
  • grunt-pagespeed
  • performance-budget
  • PSI - PageSpeed Insights with reporting

Further reading

You can find out more about improving your website’s frontend performance by reading:

The Service Manual has more suggestions about how you can test frontend performance.