The frame rate loss on popout is a known issue that’s been there since launch. There’s an extensive thread about it in the bug forum from back in 2020.
If you’re using DX12, you shouldn’t lose any significant frame rate no matter what. That’s something they fixed in SU11.
With DX11, you can pop out instruments together in that single popout window and you don’t lose frame rate. You shoudl be able to move that anywhere. But the second you separate those windows, you’ll be losing about 30 fps. That frame rate loss is even worse if you’re using vsync.
It has nothing to do with Spacedesk.
EDIT: The thread in question about this issue: