1st of all, thank you for playing the game. I hope that you liked it.
1) did you played the browser version, then it might lag as there will be lot of enemies which have been spawned. I didn't get the time to optimized the game well. I really apolozie for that.
2) The enemies have certain behaviors so if you are just out of their range, they'll start to roam randomly. This is to add some vareity.