1. Start up your game
2. Alt+Tab out of the game
3. Ctrl+alt+del and bring up the task manager
4. Find the process for the game
5. Right click the game from this menu
6. Select "set affinity"
7. Uncheck core 0
The game should run on core 1 2 and 3 (if you have a quad core of course) Anyway it didn't change anything for me, the marketplace is still a big frame dropper. But i'm actually really glad i'm not the only one with that problem, I was starting to get paranoid about my system capacity XD
