SiteSpeed follow up: Review Metrics we test with and propose official targets
We currently have a target in the Handbook for Webpages based on the Speed Index of 2s. We need to define official performance targets for this like we did for k6 as we'll need something to raise issues against now that we have a SiteSpeed pipeline.
This issue is to explore if this target needs to be updated and/or complemented by another metric like total browser rendering time with the targets set officially in the handbook.
Google, in partnership with the W3C Web Performance Working Group has actually done a lot of work in this area recently and helped define new metrics and targets that reflect modern day performance - https://web.dev/metrics/
For our pipeline in particular we care about the browser performance of our pages specifically. Based on the latest research there's two metrics that stand out as key metrics for us to monitor moving forward:
- First Contentful Paint (FCP) (<1s)
- Largest Contentful Paint (LCP) (<2.5s)
- Total Blocking Time (TBT) (<300ms)
There's further metrics that Google also recommended that I'll be looking into as well.
In particular LCP is very key as it's a straightforward way of seeing how long it takes for the most likely to be important element of the page to load. Previous metrics such as load events still have their place but tend to miss on dynamic element loads.
Speed Index does still have a place but certainly Google looks to be moving away from it as well going by their research. For our pipeline SI also includes backend, which then in turn hides purely frontend performance issues (and we cover that already with GPT). So for our pipelines we may want to move away from Speed Index.