Perfomance decrease on 'SetViewTargetWithBlend' - multiple cameras

In my pawn I have a CineCamera. The game is running with 70fps when I start it.
At a certain time I try to switch to another camera in the scene using “SetViewTargetWithBlend”. The other camera has exactly the same properties like my pawn camera

However, framerate drops to 30 and also the postprocessing set in the postprocessing-volume seems not to be active anymore. Why is that so? How to achieve this effect without performance loss?

it was my fault. In the pawn camera, the screenpercentage settings were overwritten.