I have the same issue, both at work and at home 
for me the editor can drop down to 2fps. also once I reach this slow state (10 fps or less) pretty much anything I click freezes the editor for 1-5 seconds
as you stated in your last comment I don’t think it’s about the amount of monitors, but simply the amount of windows that are open
also the more complex the main viewport scene is, the more loss I seem to get out of other open windows