Maybe this busywait thing does not apply to timedemo mode, considering that the maxfps limit is ignored there?
Don't really know, I'm not into rendering stuff so I don't know how this modify exactly the behaviour, but it seems so when reading the readme that it only affects com_maxfps:
Ioquake3 now uses the select() system call to wait for the rendering of the
next frame when com_maxfps was hit.
Anyway I think the theory of Neon_Knight is more probable as to be the cause of the framerate drop. This kind of complaints happen only on OA maps, I have yet to see such a claim when playing third-party maps.