Thanks Jeff.
Yes, it affects users who have a large number of posts. They all have 30,000 plus.
I’ve disabled all 3rd party plugins and tried to get it down to the bare minimum (docker manager, adsense and data explorer). After rebuilding I seem to be able to now get a bit further when drilling into one of the +30k users’s profile - before it would crash with a 502 error when simply trying to look at activity.
I am hoping that this is a sign that their experience posting will be improved too. I’ve asked them to carry out tests and I’ll get back to you.