@MoreBadgesThanU: We never said the wheels fall off for difficulty. We said we couldn't change it because it would make it to easy - the game designed that way, if change that aspect the game dynamic will change too much and we will have to spend another week or two re-designing all the levels.
That seems like a pretty decent spec you have there, Death2Zombies. Our lowest spec test machine is no where near that spec and it ran just fine. Are you sure you have the latest flash player? Some older versions are really bad, in fact we had some issues on a our low spec machine and a quick flash update sorted it out. http://get.adobe.com/flashplayer/, latest version is 10.3.181.14. If you click the cog icon at the top left of the screen, it will tell you what flash version you have.
@NicolBola: It has a quality setting - press Q in game (it tells you this on the level start screen before every level). We disable the flash quality setting on all games, but in this case high quality was never an option - it runs in medium by default, our custom quality changer does more than just change the quality.