I haven't seen a hint of slow down ever in my short time as a PS3 Owner, seems to be solid to me, perhaps I just missed that rough patch of early ports, but I think that old criticism of frame rates bogging down to 'unplayable' levels really needs to be shown the door.
PSN: TheGodofWine (Warhawk / R2 / MotorStorm PR)
AND
PSN: Skigazzi (for KZ2 and future games)