Many players on a server cause fps drop

I’d first look at the rendering complexity of the player models. How much polygons does it have, how complex are the materials, etc.
Then you could check if you do anything heavy in the character and playercontroller tick function, which would explain why there are fps drops when more players are in the game.

We have a big map with sublevels. When more than 10 players connect to the server and walk to different parts of the map fps drops even on clients. When we tested with 2-3 players staying in a group the fps degradation was not there. What could cause such problems?

The problem is more connected to the number of sublevels they triggered to become visible. May be we have issues with culling, where far away parts of the map are still rendered andit seems that some sublevels just stay visible even when players leave them.

Yes, always make sure, that everything that doesn’t use tick, has tick disabled, that’s huge performance boost.