Hi,
Sorry to resurrect this but I am/was having this issue too, on a Windows machine running UE5 with the DirectX 11 and DirectX 12 renderers. (Each new open window halves my FPS, from 60ish, to 30ish, to 15ish, and so on…)
Anyway, I switched the renderer to Vulkan, and now I can have multiple windows open without it affecting the FPS beyond what is needed to draw its content, meaning that when idle or flying around the scene, my framerate is 60 FPS.
This can be done by going to Project Settings > Platform > Windows tab, and then setting “Default RHI” to Vulkan.
I remember encountering this with UE4 as well, and trying to select different RHI’s manually in the DefaultEngine.ini file, and I don’t recall any of the values I attempted working. Not sure what’s so different with UE5 (other than me perhaps not seeing or overlooking any Default RHI option in UE4), but I am happily 'putin along with three screens and multiple windows on Unreal.