Maybe this should be submitted as a bug. Possibly the sim is regarding the second monitor as an entirely separate rendering space - for example imagine its a 1440p monitor and you put just the ATC panel there, FS may try to render to the whole 1440p resolution even though the ATC panel may just take up 20% of it hence annihilating the FPS; Seems very strange to me though as lots of these panels are just text and buttons - nothing CPU intensive at all. Even the Map window is not very high resolution to destroy a graphics card / CPU. Must be a bug.
I’m soon to receive a 1440p monitor to go with my 4k and if I reproduce this I’ll submit a ticket. The purpose of the ‘make into external window’ button for radio, view and map panels is to be able to bring them elsewhere but when that results in a 50% FPS hit just to move some text panel - that’s a rendering bug for sure. That can’t be by design.