There is a setting in ‘Editor Preferences’ for “Use less CPU when in background” - In the context of your screenshot, the editor might be in background mode since your dashboard is in the foreground.
What is your CPU and GPU?
If you open a new blank map what is your FPS? (on ‘High’ settings)
If the FPS is decent in your new blank map, you can start copying over aspects of your poor performance map to see when there is a big drop.
Also, one thing to check is the number of shadow-casting lights in your scene. (And if you just have 1 primary directional light (the sun 
) try turning off shadow casting on it.)