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 causes a huge performance drop? From around 70fps to 40fps.
Does this mean you should only profile when in PIE mode?

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.