Yes, it seems strange to me the fact you point out. They use Openbox by default, and it doesn't support composition in any stable releases yet. There was some work in git in a GSoC, but I don't know how far it got. I believe it's merged to trunk. I think there might be place for optimizations there.
EDIT: WHOOPS, it seems I misrecalled about Openbox, the one which received work for compositing was Fluxbox. It is Openbox the LXDE's default, though, and it doesn't support compositing.
Also, even when it's not copyed to screen, I'm ALMOST sure the content of the windows gets updated while compositing is on, even when not showing, inside its buffers. So, even when overlapped by another window, it takes cycles.