If you change MaximumSpawnTime to 4.5 instead of 5 your resulting spawn speed will be every 0.5 seconds (5 -spawnTime = 0.5 where spawnTime = 4.5). Just takes a little basic algebra.
I hope that helps and good luck with your project.
[Edit] just to be clear, you would change your Set spawnTime method to:
spawnTime = min(spawnTime + 1, 4.5)