Just to be clear, focus has no effect over the enemy attack speed, the wave 90 thing is happening because the enemies cooldown between attacks progresses based on the wave, and it is getting to a value too close to 0, so the computer interprets as 0. This will be fixed on the next version by putting an attack speed cap for the enemy (and maybe the player)
Would it be possible to switch from individual projectiles to a beam once attack speed approaches a point where it causes lag? They start to look like beams anyway, and you can set the tick rate of the beam to whatever the attackspeed is and only show the damage that happens per second instead of per tick. Ought to reduce some of the lag, I'm at wave 80 and it's unplayable because of the volume of fire coming from the enemies.