Good God...
I have never seen such a great game ruined so horribly by lag.
When the screen has to process the movement of 10+ zombies on the screen, it chugs incredibly badly and ends up trying to do all your inputs one at a time, as if they were on a to-do list for an elderly person. The problem is, this makes doing snap-decision movements (i.e. standing up, turning around, switching weapons) impossible because it still needs to process, like, fifteen seconds for eight gunshots you might've rapid-fire clicked on an already-dead zombie before it'll let you turn around to ADDRESS THE HORDE WHICH IS RIPPING YOUR BACK TO PIECES.
I have never taken off more than a point per problem in a review before, but one point does not really address the gravitas with how incredibly AGGRAVATING this lag is. This game is fantastic, in every other way, and that just provides all the more ire for the player when the most tense moments in the game have themselves ruined because your character refuses to do simple actions before it does stupid ones. I seriously tried to ignore it, even in the end of the game when I decided to just outrun the hordes rather than be crippled by lag in fighting them, but when I had a horde behind and enemies in front, and I found myself unable to either run or fight, I realized I was simply out of patience.
-1 star for some minor balance issues I might've otherwise overlooked, but -3 for lag which makes a near-perfect game unplayable.