Does using Spaces slow down the system?
your application windows are opened already (and rendered) no matter what space they are in.
if you do "show spaces" (either with "corner" or hotkey), OS X must transform each space into the "reduced" form so you can see it on-screen.

If you have 8 windows in 4 spaces, only 4 spaces will render
if you have 9 spaces, the OS will render 9 spaces (and the same 8 windows).

each Space eats additional memory


I'm using 9 spaces and here is my own experiences:


There in NO difference if you have enough memory (no swapping to HDD to render everything)
There IS a difference if you don't have enough available memory to avoid a swap to the HDD (perhaps too many apps are open). In this case, when you do "show spaces" there is an increase between showing 2 spaces vs 16 spaces (but nothing that is really bad or terribly noticeable)


So,


spaces does not slow down your system noticeably (in the most cases), but
definitely eats some more memory (but a small amount), so you may get swapping a little bit sooner, and swapping is that what is really slowing down your system.


ps: these are my experiences, others may have other opinions.