There are no data points between 28 April and 12 May, that's suspicious as well.

Is this real-user navtiming, webpagetest, or webpagereplay? What does the rest of the main dashboard of that look like for the same time range? If visible on synthetic, are there notable difference in the screenshots of the before/after runs?

Do you see it on other platforms/wikis/articles? Which one not? If only on some, then look for a possible correlation with on-wiki change in content or MediaWiki-namespace change (site scripts).

Does it correlate with a sync-wikiversions annotation when you zoom in? If so, check SAL to see which train branch and wiki group it was.

Haven't had a change to jump into this.
Here's the graph showing impact on first paint etc.. Hopefully it's just an issue with the metrics and not a real problem.

https://grafana.wikimedia.org/d/000000205/mobile-2g?panelId=90&fullscreen&orgId=1&from=now-30d&to=now

I'm pretty sure this relates to 57b9be1beff27b18c582371f3703d949607bea14 and T250730 looking at webpage test (http://wpt.wmftest.org/result/200428_BH_83/1/details/). The date matches.

It seems various oojsui modules are now being pulled onto every page load. That wasn't the case before the spike. If it is this, we should restrict those styles to the special page that needs it.

Before After

I don't think the aforementioned patch (57b9be1beff27b18c582371f3703d949607bea14) is related.
The relevant styles are only added for that specific special page (see extension.json & SpecialMediaSearch.php)
But more importantly, that extension is not enabled on enwiki (see InitialiseSettings.php and Wikibase.php), so whatever code in it shouldn't even get executed.

Thanks @matthiasmullie for looking I'll look into this some more and see if I can work out where these new modules came from.

On closer look this morning this is the infamous quick surveys bug - T159738

The following modules have been added to the page and the rise in kb matches.
ext.quicksurveys.init.lib
oojs-ui-core
oojs-ui-core
oojs-ui-widgets
viewport

Sorry for the noise. This must be the 4th time I've looked into that bug now!