I have Dark Souls: Prepare to Die Edition on steam. I have had it for quite some time and actually have over 100 hours in the game. I used to play it a while back, I even used DSFix and other mods that made the graphics even better then they usually are and it never, ever lagged at all.
The lag is a little strange. Sometimes it will run well and other times it seems to just lag like fuck and I only get like 20 fps (which is VERY annoying for me). Sometimes I can fix the lag by looking straight at the ground or resting at a bonfire like 5 times.
My Specs:
windows 8.1
geforce gtx 750
16 gb ram
intel core i5-3350p 3.1 ghz
(anything else you need to know just ask me)