Ok, some more on the topic... new server is here, 8 cores, 16 GB ram... all working fine, APC(u) is up and running.

Had to rush switching from Dolphin to Una now for reasons, so Una is live now. All working well, server is mostly idle, cache is up to 1.5 GB of ram, all fine. But the community (=UNA) is partly completely unresponsive. It's like clicking a link and then 20-30 seconds nothing happens except browser showing a spinner. And we have around 6-10 people online from 8k. 

After that "dead" time which seems to happen for all users at the same time (server is at 0% load meanwhile) it shortly goes up to like 90% on all cores for a second (probably executing all the commands been on hold so far?) and then it works smooth and fast again until the next hickup. Can't say when, maybe next click, maybe in a few mins.

Alex T⚜️ Any idea what that could be? Where to check what?