Good game! I loved the voice recordings. They were a nice touch.
I played on linux and had problems with the performance going down when there was some enemies on the screen. In the case of this affecting multiple users, this problem may be due to collision checking. I had this same problem in my game when I had 6+ enemies in the game and the way I found to mitigate it was:
1. checking collisions with what is on screen only
2. reducing the number of wall colliders (in the start of the level I grouped the colliders of the walls).