In response to a review, I have increased the minimum spawn rate from 0.2 to 0.5. This means the fastest enemies can spawn now is every 0.5 seconds. This should improve performance at higher waves, as well as make the game a bit easier.
I used some design choices like this to see how much I could push using the crap the load of optimizations I used to make this game even possible... I've looked into improving the loading times, but it would take a bunch of re-writing as each pooled object would need to be baked into the scene, which simultaneously increases the game file size. Maybe I'll get around to it sometime if the sales start to pickup.