I'm not sure, I've never used it myself, I just know about it. I'm guess it has a marginally small effect on the performance, other members will be able to give more detail on the program.
Or a quick search of the forum should bring up a bit more information.
There are a few detailed responses from the past but I haven't noticed much slow down. There is a little I think, but I've used Shapeshifter pretty much ever since I got my Macbook and I don't remember it being any faster.