The lag seems less of an optimization issue and more of finding some sort of memory leak or compounding process. The game runs fine at first but the longer you play the worse it gets. (A page refresh will get rid of lag temporarily)
great game though you don't have enough enemies that can deal with archer spam I was able to complete most the game by just using about 100 archers and maybe 2 drag/peng (also I only counted 11 puppies in end credits)
to get the easy achievement well easily just play level 1 and 2 and kill 2 seekers each time. Because the kills are cumulative between games just start new game after level 2 and it should only take about 10 min (3 games) to win.
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