It is lightning fast, and much faster than the Nexus S on stock 2.3. I know because I tried them side by side.
The reviews are correct though, there is some stuttering. I'd say it's annoying as hell, but I'm an impatient guy when it comes to tech-gadgets. I'm sure it's only software issues that will be addressed, either by LG or by custom ROM's.
Sometimes it takes about 5-10 seconds to open the apps list when adding them to the home screen, and sometimes it's as quick as expected.
Sometimes it will stutter while scrolling but most of the time it's smooth as butter. You can clearly tell that there's nothing wrong with the performance because of the smoothness, but there are undoubtedly some serious flaws in the implementation causing intermittent stutter that needs to be addressed.
Playing games is as smooth as I've ever seen. It's simply amazing; There's no stutter or lag AT ALL, so the stuttering you see in the menus seem to be fairly isolated. So far I have tested Angry Birds and AB Rio in addition to Fruit Ninja + Samurai II from Tegra Zone. Samurai II on the TV looks better than a Wii game and is closer to PS3 to be honest.
Another evidence of the incredible performance is the Gallery app. My jaw dropped to the floor when I saw how quick and smooth it is. My Nexus One is like an old Sony Ericsson with Symbian in comparison. If you haven't tried an SE phone with Symbian, consider yourself lucky.
HTC One X | ASUS Transformer Pad Infinity
CyanogenMod Release Notifier: Python 2.7
[ Free Dropbox ] - 2 GB + 250 MB bonus