spawn rate outside of screen and despawn out of a certain screen distance would improve frame rate
do you use an ememy pool? you should adjust it a bit
the frame rate sucks badly in html when playing for a long time. maybe despawn enemies very far from the camera, make enemies spawn quicker and off screen or use a enemies pool and make it up to 500 enemies.
also when you lose it should tell you how long you have played
overall it's a beautiful survivors genre game.