[Slick2D] Extreme Lagging

Even more fun with this annoying errors within this game. This time it’s duper annoying and super game killing lag! Could someone please just review my source. Then tell me whats wrong, and what i should change.

Download: https://www.dropbox.com/s/6szr0uway97qcjd/Zombie%20Nauts.zip

I tried to fix the most things i could. It had no effect. So, I hope you more experienced developers will figure out what the problem is. Thanks in advance!