It's faster (in general) because Compressor is 'network aware'.
It shouldn't become sluggish, period though. It may hiccup if you have sent an FCP project and you click on it (while it communicates with FCP). That can be a second slowdown (but again, higher in quality.)
If you're doing a straight transcode, MPEG streamclip is fine.
Best,
Jeff I. Greenberg
Editor/Author/Speaker/Consulting
My NAB seminar schedule, contact info and more