Alright, I made some tests with the framerate. Interestingly the problem seems to stem from Unity in general.
I have set out a 4 page example project, with a black taskbar at the bottom, which switches between the 4 pages.
The FPS is constantly on 60 FPS, but when I switch to the page with the camera scroll view, it dips down to 55.
It seems that the problem lies in the cameras clear flags. If a camera is used as an overlay with "Don't clear", the framerate decreases noticeably. Only on iPhone 4 though, the iPad 2 can handle everything at 60 FPS.
Is there anyone else, who experienced the same problems with multiple cameras on the iPhone?