I posted this on XBH a while ago:
I use Bayonetta to check if the lag is present or not. There's a chapter (Route 666) that always causes lag, and it doesn't matter if you've been running the game for 10 minutes or not. I tested this on a Xenon running XBR_1. Tried XBR_0, but that didn't seem to make any difference.
NXE install -> lag
Straight from DVD, only MC present -> lag
Removed DVD & SATA-HDD, launched XeXLoader from MC and ran game from 300GB USB ->lag
SATA-HDD present, launched XeXLoader from HDD and ran game from 300GB USB -> lag
On a side note, another game, Samurai Spirits Sen (fighting game) only lags during the loading screens, in-game it's perfect. And the Bayonetta chapter seems to be streaming data heavily from the storage device. I would think AC2 streams in-game data as well. Gonna try SFIV and Tekken 6, I'm fairly certain those two would be perfect in-game as well, since the data is presumably loaded into RAM before the fight begins.
Edit:
SFIV lags in-game after a few minutes. I have no idea if it's streaming data or not when the lag begins.
Tekken 6 stutters a bit during loads, perfect in-game (HDD seems silent during fights). I should add that I run these from the 300GB USB drive using XeXLoader.