That was fun! I could've used an option to stop shooting, though, since I managed to cheese the waiting part by finding a spot where few zombies could find me. Not having to move could've been better for my eyes since the FPS got so bad there (I read your comment about optimizing below, and I agree that it's most likely the pathfinding that causes the lag). The voice acting was excellent, though! :D
If I hadn't made this for the "Out of Control" theme, I probably would have just gone with a click-to-shoot control scheme. I did try and make sitting and waiting a less viable strategy in general, though: that's why backing into a wall screws up your aim so badly.
I think the game runs most smoothly if you tend to stay in the centre of the city. I noticed that relying purely on pathfinding caused an enormous amount of lag once you got far away from a few zombies (I assume because they have to plot a route through the entire area to get to you) so my solution was to have them try to make a beeline initially (with the buildings laid out so as not to funnel them into too many dead ends along the way) and only start looking for a valid path once they're within a couple of thousand pixels of the player. This setup means you can't escape a horde by letting it run face-first into a wall while chasing you, but unfortunately I think it may also cause performance problems if you do manage to get a large cluster of zombies stuck somewhere. Getting close enough that they all start looking for you at once is likely to send the framerate through the floor.