It’s not prefs, or the card, or the ram, or anything other than FCP6 neeind to have this fixed. It is only utillizing 100% of the processors instead of 180%-190% like it use to use before FCP6. It’s the code.
I’ve had the same problem with a 4x3ghz Mac Pro 1900xt card, 4GB ram, Xserve Raid. And takes 7hours to render an SD 58:30 with 20 .motn files overlayed on the timeline. Use to take 1 hour.