Remember, the PS2 was supposed to be "100% backwards compatible" and it turned out it wasn't.
Plus, the PS3 is running on an architecture that is night and day different from PS2 and PS. I doubt highly that they've managed to multi-thread all of the old games, and I doubt even more highly that without multi-threading the Cell processor can run anything even at PS2's speed.
PS3 is an "In order" processor, meaning it can't look ahead and start processing something in it's spare time. PS2 and earlier games were "Out of Order" processors, giving them a speed advantage.
PS3 is competent only when code is in order and multi-threaded. Which legacy games aren't, so I doubt they'll run at speed if they run at all.