Are you in DirectX 11 mode? In this mode Fusion doesn't change the resolution (I couldn't find a way to do it yet with the current structure of the display in Fusion apps...), it stretches the display. But stretching a HD image to 4K takes time even for a GPU. This is the main thing we still have to improve for DX11.
In DirectX 9 mode, it changes the screen resolution, so this would be probably a bit faster.