Track performance impacts of built files

[romaricpascal]

What

Monitor metrics to judge how our built files may affect performance of projects using govuk-frontend. This includes the files in dist as well as those inside package.

This should be as automated as we can to save us the time of gathering and comparing these metrics, but could be implemented progressively.

Why

While we do a diff of the files in dist before releasing, it'd be good to keep an eye on the size and contents of both dist and package, so we can spot unexpected variations closer to when we make the changes.

We want to be able to see the impact on end users – people viewing GOV.‌UK and using services – of any changes we make, and as a way to catch any potential problems, e.g. we've increased built filesize by 20MB.

Who needs to work on this

Developers

Who needs to review this

Developers

Tasks

Earlier discussion

• Get visibility over web performance metrics #2076

Investigative spikes:

• Investigate ways we can keep track of perfomance metrics in PRs #3051
• Gather performance stats with rollup-plugin-visualiser #3188

Implementation steps:

Before v5 code changes

Beta Give feedback

1. Introduce reusable action/workflow/script for commenting on PRs reusing the same comment #3545
   2 of 2
   github-actions javascript tooling +3
   
2. Upgrade Rollup #3462
   javascript tooling +2
3. Gather built files sizes and display them in review app #3280
   javascript performance tooling +3
4. Gather breakdown of modules content #3281
   javascript performance tooling +3
5. Display file sizes and module breakdown on pull requests #3282
   javascript performance tooling +3
   

Post v5

Beta Give feedback

1. Compare file sizes between branches #3284
   javascript performance tooling +3
2. Compare breakdowns of modules between branches #3285
   javascript performance tooling +3
3. Break build when change impacts on file sizes/modules content is too big #3286
   javascript performance tooling +3
4. Measure impact of transpiling #3287
   javascript performance tooling +3

[stevenjmesser]

Moving this epic to the backlog so the individual tasks can come through the board.

[colinrotherham]

Going to move this back to the Backlog for post v5 tasks