I created a video showing performance of switching in and out of full screen in Aperture in Snow Leopard and in Lion. It shows that it is virtually instantaneous in Snow Leopard and quite sluggish in Lion. This is using my 2.8 GHz i7 iMac.
http://www.youtube.com/watch?v=955aoEHKKQ4
http://www.youtube.com/watch?v=955aoEHKKQ4