I don't think it's the rain that's creating this problem.
I was logged in with a couple of friends - at either end of the airfield. They were at the air traffic tower, I was near the tents. We were all fine.
As I walked towards them (they stayed where they were), as I reached the fenced off courtyard area next to the fire station, then screen juddering started.
If I then backed off, it would then go smoothly.
When we got together, one of us logged off, and the frame rate of the others then increased to a playable level, then when logging back in, it dropped through the floor again.
When just 2 players were together it seemed to be fine. 3 and it drops.
Related?