Thanks for your answer MMBovo!
Actually, about one hour after my post the server stabilized itself and the game ran smoothly again. And since that time we never had any problems again.
But if it will happen again, i will take your advices into consideration ;)

Hey guys,
whenever there are some buildings built in a certain area (not tooo much, but quite a bit, a small village, like 20 chests, 20 tables..... in about 5 houses) the game is getting extremely low FPS in that certain area. From normally 100-200 FPS it drops to only 7-10! Graphic settings won't affect anything at all. As soon as we leave the area and the buildings dissapear from the render distance, the frames are coming back...
All of our 5 players expierence the same problems, so we are pretty sure it should be a server issue...
We rented a server on a Hosting-Company (GPortal, if anyone cares), but first we tried to host an own server on an own (seperate) PC, which caused the same problem after our town reached a certain size. (Wich is why we bought a server for more perfomance, only to get the same issue again :( )
It may be worth mentioning, that the server only has about 80% CPU usage when starting and then it won't ever use more than about 3%. So i thought, maybe the so called "WorldTickCPUMax" setting is bottlenecking the perfomance (currently on 0.1). It won't let me change this setting from side of the server company appereantly (support already contacted) but on my own PC-server it changed nothing at all......
Has anyone the slightest idea what to do about that? What are the dozens of servers with HUUUUUUGEEEE metropols, monuments and vehicles doing, what we are missing? I can't imagine that these guys are playing with constantly 5 FPS :D ;)

Thanks for your help guys ;)