Great game!
Awesome game dude! Although you could have made the upgrades juuuuust a hair cheaper, and allow the turret gun to damage buildings as well after say, level 3 upgrade?
As far as the lag issues go. People should go into task manager and shutdown anything abnormal to their system. In particular "plugin-container.exe" Killing that specifically will cause flash to restart (and make you reload the page) but will fix the lag issues. If you still have lag.... upgrade. I'm running a C2D @ 3.0ghz. Nothing special.