Yeah, you can alter the percieved game speed by doing a number of things.
1) Reduce the calculations. I don't like this because it messes up precise physics calculations, especially non-linear calculations.
2) Change the manual sync rate. I use this: only call SYNC when there is enough free SCREEN FPS() time to do so.
3) Do complicated, unstable (at least in my experience) calculations with TIMER() and manual SYNC rate (or calculation reduction).
There are probably more ways to unlag, but this is all I can think of.
I don't think that checking SCREEN FPS() slows the game down. Try setting SYNC RATE to 0 and see if SCREEN FPS() slows you down any. (You could get a precise measurement of FPS with TIMER() to compare to the SCREEN FPS() if you wanted to compare calling the FPS function with not doing so, but doing so would be probably very difficult). My theory is that setting SYNC RATE will try to push the FPS below the value you supply, but will do so in a rough way (not enough time increments) that may end up pushing the rate below what you actually desired.