Thread by @TheRealNooshu

🧵As I mentioned last week GOV.UK removed jQuery as a dependency for all frontend apps, meaning 32 KB of minified & compressed JS was removed. So let's see what difference this has made for users by examining our RUM data. Thread will mainly focus on JS CPU time.

A good place to start is users on low spec devices. The Universal Credit sign-in page shows a lot of this traffic, as seen in the images below.

We see many of our key metrics trending down (for p75) after the change, including frontend time, First CPU Idle, JS Long Tasks

If we compare before / after for our JavaScript performance, we can easily see the percentage improvements:

If we look at the extreme end (our P95 users). they also see significant improvements:

If we remove the page label restriction and look at all pages RUM data, we see a similar drop across all pages for users at P75:

This improvement is still seen across all pages at P50 (median):

If we examine only Android users across all pages at P75, we also see improvements to JS Long tasks of 7%.

If we restrict it to only mobile devices at P50 users across GOV.UK we see a 10% improvement in JS Long Tasks:

The original tweet I'm referring too is here:

And there have been a lot of questions about the graphs, we use @SpeedCurve both for RUM and Synthetic performance testing.