Thanks to a comment by @[email protected] , I checked and saw that 'Federation debugging' mode was enabled. I had enabled that when the server just started (less than 3 weeks ago) and I had an issue with federation.

I thought I had switched that off again, but apparently not. This mode causes the federation to be done in the foreground, so your 'Post' or 'Comment' action will wait for that to finish...

This solves the most annoying issue, and makes the site way more useable. There are many other issues, but we'll get there.