When you look at a crucible with astral power, then pause your game for a few seconds, and unpause - you can see a lot of particle spawning. Maybe this is caused by particle spawning being dependent on system time or some kind of external timer instead of in-game client ticks. In theory could cause lag when you return to the game after long period of time, but not sure about that.
A bit strange to see that anyway

This is an issue that's been there for a long time, I think it has to do with some particles originating on the logical server.

Fix in the upcoming 1.21 port.