Cool game! All the base mechanics are well-implemented and I always appreciate the effort of putting in a tutorial :)
I think that the FOV could have been a *teensy* bit wider to see the enemies easier, or if anything, have the FOV slowly increase every wave so the play can see more and more enemies at once coming at them, scary!
I am curious as to what was causing the FPS drops, was it spawning or the bullets/enemies not deleting off screen?