Running a Q9550, which NFS Shift uses around 50% of at most, so I don't think it's that either.
I'm not sure, while overall cpu usage isn't MEGA high on my i5, it was giving at least 2 of the cores a reasonable amount of load. I could see a E5200 @ 3ghz potentially bottlenecking it in places.
The duel invitational between the Ford GT (my car) and the Viper is the devil incarnate! I must have re-run the deciding race aver 50 times in "pro" mode before I had to let it go...
Initially I found that fairly hard, however revisiting it after getting more used to the game and it wasn't so bad. Basically you just have to make sure you handle that right-hander properly coming over the hill i.e. don't let it slide out wide into the gravel. In the last 3rd of the race you can make up a lot of ground as the Viper slows down too much on the last few bends, you can just blast right by him.
Failing that I always find roughhouse tactics work quite well in the duels, if you spin them out you can often pull away. The Viper does seem tougher to take out than some cars though.