Hello, I have a similar or possibly the same problem; there seems to be a slight but noticeable input delay of around .15 seconds. At least for the mouse, I can't really tell if it goes for the keyboard input as well, with the inertia and all that. Mouse smoothing on/off doesn't affect this.
The game is still playable like this but it would be nice if someone knew of a way to solve it (as this is one of my all-time favorite games).
(Has happened to me with other copies of Unreal Gold too, not just the one I got from GOG)
The problem with UT is that it plays better in opengl, for me i've always found that if i manually change the hardware mode to this then i get better looking gfx & input on the controls.