Hey there, I apologize in advance for this post, as I will not be able to supply most of my system specs, but will do my best to create an image of my system's capabilities.
I just recently bought Demigod (last night) with my girlfriend, after rushing home around midnight to get it installed and try a game before calling it a night, I finally do just that. I'm sitting on my usual (albeit getting older) gaming machine, basic 2gigs of ram, capable of playing Warhammer Online, Lord of the Rings Online, with VERY minimal lag (and even then only during zergs in the pvp moors), and never having experienced a moment of lag in WoW, I can play most every game I've tried without any significant slowdown.
My girlfriend is sitting playing on a spare machine I have, a single gig of ram, and nothing noteworthy, it experiences lag from time to time on pretty much *every* game, and is an all around worse machine than my own.
We both go to single player, start an easy arena. Takes a bit to load in, but no worries, first time I've run the game.
Screen does some flippy-rotating jerky sh*t (Assumed at this point in time to be a "welcome to the match" animation).
I promptly drop to around 5fps, and while able to situate my "hero" along a causeway, am really unable to move or react quickly due to the sheer amount of jerkiness, and general "lag".
However, girlfriend's piece of a machine, other than dropping fps a little in a "war", seems to be perfectly fine.