Confirm, exactly the same for me. Except that switching from window to fullscreen dropped my FPS by half instead of increasing it. W520 Q2000M Nvidia with newest available drivers (2018).
I have the feeling the problem eased a little bit over the last patches (means it takes longer until unplayable values) but this can just be a sample bias.
Concrete example just now. Docked in a Citadel in Perimeter market window open, planet in background … smooth 60 FPS. Undock, fly to Jita, fly back, dock again. Same scenery/angles/zoom … max 54 FPS.