It might be a good idea to add a setting that removes dead bodies from the ground instead of them staying on the ground. It would probably help with the lagg. There could also be a feature to remove blood/gore and bullet casings+bullets to reduce lagg. The AI on your teammates could be a bit better, particularly when an enemy is preparing to shoot me (when the big red reticle closes in on you) and when a zombie grabs me. If they acted to help me during tight situations like that more often, I'd appreciate them a lot more. I do respect some lack of intelligence though, since they can't do everything for me.
Really It's an amazing game in all other respects. Just needs a bit of work...