Is there a way to fix lagging when I get loads of those little guys blue and green one and those with yellow hair.
I have perfect defence but when it glitches its like I did not place any cannons. I don't see when I get hit or cannons slow down. So I lose nevertheless and can't even buy stuff fast enough because it lags.
I even lowered the quality but still not working.

You can right click and put the resolution on low, turning of the music helps, and having no other running programs helps XD

On the later levels I just leave the game running in a tab and don't watch it. Just buy new guys between levels if you want to and set it off again.

Sadly there isn't too many ways (other than what has already been said) to reduce the lag. The game is pretty bad for it. A lot of the enemies skip straight to Mr. Mayor's house. :c

yeah, I do lower the quality but still as you said some skip over to the mayor house.

Best tip I've ever heard is switch tabs on the browser. You wont be able to help, but it keeps them from glitch-teleporting.