Well it does background operations when you're not clicking around/dragging stuff. But processing will start once you leave the application idle. I set mine to start after 0.1 seconds.
Either way, I had my activity monitor running, and it's shocking how much processing it's doing, even on color corrections, which should be real-time especially with optimised media... plus media management of media is better, but management of renderfiles is a huge cluster fudge. Background processing can be turned off entirely so that effects happen in realtime, or at least without it rendering.