In T260860#6426584, @Gilles wrote:

I recommend that you track performance from the user's perspective. For instance, the time it takes between requesting the special page to them being able to see all the elements from their global watchlist. This can be done fairly simply with mw.track. The data will then go to EventLogging and be easily displayed in a dashboard. Looking at the high percentiles, you should be able to see if there are some serious outliers worth investigating. You should track as many timings this way that you think are relevant, for instance you might want to track "fast mode" separately.

Event names (based on other extensions):
timing.MediaWiki.GlobalWatchlist.firstload.display.normal and timing.MediaWiki.GlobalWatchlist.firstload.display.fastmode
(display corresponds to using GlobalWatchlistSiteDisplay, vue is for GlobalWatchlistSiteVue in case that loads faster or slower when it becomes available)