Frame rate drop when panning / looking around

Since I updated to 4.18 my frame rate drops dramatically to about 16fps whenever I pan in the editor. When I am moving in any direction however everything is fine.
Also when I go into game mode the frame rate drop doesn’t occur.
This happens even in the empty default levels. So it is not a matter of hardware or level complexity.

I am using unreal on the 2017 base iMac 27 inch.