Yes, as dynomutt said
Also, move in diagonals. Then you move faster and first monsters can't kill you that easily.
I finished the grenades guy after I've bought a considerable amount of power-up in the main menu.
Or maybe weapon effects reduce fps as well. I was playing a guy with grenades, it was really hard to kill anyone, but still the fps was bad to the end of the game.
The game is pretty addictive, but the performance is terrible (Chrome). The more enemies you kill, the more the game stutters. FPS become very and very low.