Second this issue.
I am having same problems, however, it seems to be fixed when setting Graphics Quality to Medium or High. The game is completely unplayable when setting Graphics Quality to Highest.
This problems is definitely not my hardware or drivers as I normally can play any FPS on high settings with 1900x1200.
System summary (dxdiag attached):
Vista 64 with latest SP
Radeon 6770 with latest Catalyst drivers
Let's help TT fix this so we can enjoy this on the best quality.
Other than that loving it so far. First episode was very short (even with issues, finished it under 2.5 hours). Overall, great production quality.