This demo is out almost two months before the game is set to be released, so you have to figure the devs are still working on it. If it's true that this was an inadvertant leak through a mag, then their build is probably at least a month older (figure it takes time to get the game to the the mag, get it pressed and back in time to be inserted).
It's not like UT 2k3 where the demo came out after the game went gold, or NOLF 2 where it came out after release, so maybe they intended it to be more of a mp beta than a demo...
as for myself, I didn't notice much lag on the disk game. I'd like to see them slow the disk down a touch, but the mp certainly has potential, particularly since it is different than anything else out there...