I had this issue for most of the day yesterday where it was ridiculously laggy between updates. I would literally tweak a setting and watch youtube videos on another machine while waiting for the refresh to be done. I saw that I had some textures using absolute paths so I had to redo them and when I started over it was much better, but it's still laggier than I like  (about 1-2 seconds) considering I'm running a 6-core processor @3.4ghz, Nvidia 580 gtx 3gb, and 32GB of RAM.

Only item in the scene is one character that I'm doing materials on.