Is my game's performance going to be bad if I use several terrains?

Unity’s Camera is acting very weirdly and I can’t set up the RTS like camera properly.
If I resize the terrain then that seems to do nothing. It resizes itself on the “Scene” window, but not on the game Window.

Is it a bad idea to use several terrains?

It depends, i’d say that it most matters with how high resolotion the terrain is, so if you have one terrain that make up 512512, and then 4 that takes up 256256 it doesnt matter… If you change the terrain resolution the size grows, so the resolution seems to be fixed at some size in unity space, and what scale ur using in the transform doesnt matter at all.
try going into the settings for the terrain component (far to the right) and change resolution there.
if that doesnt work:
You said that you’re struggling with the camera, perhaps you wanna use the rts camera from assetstore, or else try changing the perspective/orthographic view.