Go to Settings section, and select Displays. Now, you'll have to do this to each display you have, even the main menu. Select the display and go to Layout Options. In here, you'll find Enable Monitor Static Effect turned on by default, turn it off.
I've been using the Cosmos theme, which is quite demanding. Cosmos theme, without snaps or boxart/cartart, is around 15 to 40% or so. Well, with snaps and boxart/cartart, it doesn't change, same cpu usage.
On the default themes, Basic, Reflect, Attract, etc.. You'll get 5 to 10% of cpu usage with snaps, same without snaps.
In terms of threaded optimizations, there is no difference for me. I tried it, and it's even worse, constant 100%. If you have several threads, I think you should leave this option to Auto.