I never used workspaces, so I have set number of workspaces to 1. After restarting, unity seems to be kinda snappier.
What`s in theory?
With anything resembling modern hardware or more then 1 Gb of ram you are not going to notice a performance hit.
If you wish, run top in a terminal and add a work space. Top will show you the changes in ram and cup use. You will likely see a bit of a change as you add the workspace, or if you switch workspaces. Let it settle ;)
So yes it will use resources, but the affect on "performance" should be negligible.
If you do see a hit on performance, then you would have a bug, probably in the video driver / graphics card.