Unity camera render performance with many terrains

Hi, I am currently building a tile based game where the tiles are represented by unity terrain objects. Unfortunately the performace goes down linear with the amount of tiles I generate. I identified the process Camera.Render to be responsible for my bad performance but honestly this is where my knowledge stops. I already activated Occlusion Culling on my cam so it shouldnt render objects which arent in sight right? Still the performance is equally bad if I look in the distance or just on a few tiles right on the ground. If I disable the camera my game runs just fine which in my opinion shows clearly that it is not because of other performance problems. I think that for some reason all my obects are rendered all the time, even if they are clearly not in sight. Please help :/

EDIT: I just found that of my 99,5% rendering 99,2% go into "Culling" of which 83,9% go into something called "PrepareSceneCullingParameters". The 2 subprocesses of that however show no significant workload. So obviously something culling based is going on but I dont know how to optimize this.