It becomes sluggish like every other processing-intensive
filter becomes sluggish. You may need to pre-render
chromakeyed sections, or live with the lower frame rate
until final render. In either case, final program will be
fine.
vegas has a excellent Chroma key .. yes it may be a bit
sluggish BUT that is processor speed NOT the quality of
the rendered keying ... i find the next best chroma is
either after affects or doing it frame by frame in
photoshop... try getting them to play it back without
rendering !