Low FPS when multible windows are open

I have the problem that when I am in the editor and open another window, the fps sink. (It happens even when windows from the engine popsup like “new plugins are available”.
every time i open the newly opened window as a tab in the editor window, the fps rise again to max.
So it only happens if more than one window is open
also it first started 6 months ago and it happens in every version of the engine including the “old” versions.
I have a 8700k 5Ghz and a GTX 1060 6G and I do not think it’s because of them.
Thanks in advance.