How to get rid of the slowdown
Okay, the big problem was the low frame rate. Levels take an hour or two on a typical home computer, they take more like 15 minutes on a couple thousand dollar machine capable of withstanding big flash.
Since most people surf with five year old PCs from wal-mart, please consider the following:
-Clear all of the fine details out of factories. All of those lines add up, you get into the thousands before you know it.
-Take a few shapes out of the turrets. Instead of having a gun be five rectangles, have it be two.
-Remove the soft edges on the smoke. The smoke is really complex and detailed. It looks awesome, but flash hates complex and detailed. Cartooney-looking hard edges will slow the game less.
-Ignore all feature requests. This game has more cool stuff than the average turret defense, adding more complexity will worsen the slowdown.