I don't know if this is late to the game, but from the perspective of one who uses the http API: in the past 2 weeks or so, on en.wikipedia.org, a multi-user "usercontribs" query went from always returning immediately (sub-second) to always timing out. Is this change intended to fix that?

Your issue isn't really related to this task. But I suspect it'll be fixed by https://gerrit.wikimedia.org/r/c/mediawiki/core/+/461440, when that gets merged.

I was sent here from a thread in mediawiki.org API talk:Usercontribs. On the English Wikipedia, in recent days, a simple usercontribs list request with two ucusers has changed from always responding immediately (sub-second) to always timing out. Here's the actual URL I recently tried:
GET /w/api.php?action=query&format=xml&list=usercontribs&ucuser=DavidBrooks%7CDavidBrooks-AWB&uclimit=20&ucnamespace=0&ucprop=title%7Ctimestamp%7Ccomment%7Cflags&continue= HTTP/1.1

I thought that might be the case. Just wanted to be sure you were aware.

Repro steps work - thanks for the quick fix.

Confirmed: I just downloaded version 5700. When trying to switch projects, the UI says it cannot load Newtonsoft.Json, version 7.0.0.0. The version in the download zip is 6.0.8.18111.