On slower computers, the game runs slower (obviously). Enemies still spawn at the same rate though, which can lead to them clumping together, making it harder for your turrets to shoot them down, especially air.
I found if you alt-tab to another full screen window, the game will run at its full speed, making those last few waves pass quicker. A speed up button would be nice though. Also hotkeys for towers, upgrading, and selling would be nice.
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send a screenshot pls.