I didn't have no lag problems until I started on lvl 15 and I didn't even have any of the higher up weapons yet. I went back to lvl 1-10 and it even had lag then. I don't think it has to do with what weapon we are using, I think its the number of enemies spawning per lvl after you get near the end of the game.
 "Flight"){kind=icon}
 "Vital Bloodshed"){kind=icon}
 "The Last Stand"){kind=icon}