LR3 has (also) improved in terms of speed, compared to LR2. That's why I would suspect that when LR2 has an acceptable performance on your machine, LR3 will run a bit better. Especially the 64 bit version makes a remarkable difference (on Wintel computers at least).
Probably that's why your are asking, but I pose the question anyway: Did you already test LR3 on your computer?
Thomas