Mouse pointer location in editor affects performance

Can any performance experts explain why moving my mouse pointer from the view port to another window in the editor (especially Content Browser) causes a huge performance drop? From around 70fps to 40fps.

Another head scratcher, if I go full screen (F11) when not in PIE mode my performance increases.

My takeaway is that you pay a huge penalty when using ‘realtime’ mode in the view port and that you should always be in PIE mode when profiling.