It may potentially be worth noting that this can also sometimes occur when the target user hasn’t actually edited that many wikis — my GlobalContributions takes a looong time to load, even though (according to my CentralAuth) I’ve only actually ever made edits on 8 individual wikis.

Without having looked at the code, my guess is that this could potentially be related to having (relatively recently) run the Global SUL tool - and, therefore, that CheckUser might potentially have entries relating to account autocreations on a larger number of wikis. If my guess is correct (& thinking out loud), I wonder if there’s a way to prevent any permission-checking API calls from being made to wikis where no actual edits have taken place? (I apologize if my guess is incorrect and/or of no relevance!)

There's some discussion in T384717: Investigate external API call error on Special:GlobalContributions about the permissions checks. The solution for T384717: Investigate external API call error on Special:GlobalContributions should solve this task too.

Change #1128435 had a related patch set uploaded (by Máté Szabó; author: Máté Szabó):

[mediawiki/extensions/CheckUser@master] GlobalContributions: Use higher concurrency for foreign wiki requests

https://gerrit.wikimedia.org/r/1128435

I think I'm going to resolve this for now as looking at instrumentation we look like we've improved times from when we started:

I think there were some concerns w/the concurrency patch ⬆ so it sounds like we'd prefer to maintain status quo if possible and as such, there area no more immediate actions planned for Special:GC performance improvements. We'll keep monitoring performance and if this recurs, we can re-open.

Change #1128435 abandoned by Dreamy Jazz:

[mediawiki/extensions/CheckUser@master] GlobalContributions: Use higher concurrency for foreign wiki requests

Reason:

Change is -2'd, written by a former employee, and attached to a since closed Phab task

https://gerrit.wikimedia.org/r/1128435