I've always had the affinity set to use all the cores on my quad... but I used to have REALLY bad boat lag to the point that I couldnt use boats (by the time it loaded, I had already gotten to my destination AND the boat was leaving again). What solved this was that I changed from using 5 folders of WoW, to just using 1 Folder for all my instances of WoW. Not sure if thats your problem, but its worth a try.