I can back you up on that.

On a X2 3800@2.75 with 4GB of ram (2T) I was getting the following:
- 2610: ~37min per frame => ~590ppd
- 2651: ~24min per frame => ~1050ppd

That's a hell of a difference. I think 2610 is heavily dependent on cache and ram timings. 512KB cache and 2T ram doesn't help.