Hi, your game has a couple of issues but is otherwise functional and potentially fun.
Having to click once for every shot gets a little tiring, it would've been better if holding down the left mouse button would cause the player to fire continuously. You die a little too abruptly and the score doesn't get cleared when you respawn from the game over menu.
The biggest problem however is the amount of enemies that spawn in. The game spawned in enemies until the arena was almost completely full making it impossible to avoid them, at this point my framerate would get really low aswell. I noticed there were fewer enemies in the screenshots than there were when I played which makes me think that maybe the rate that enemies spawn at is tied to the framerate.
On a more positive note, I like the design of the enemies, they remind me a bit of grow home and some old flash games like box head.