As Bernat pointed out, the game starts out running ok but slows down as the game progresses. The issue is not actually the progression of the game, but it is in fact related to how long you have been playing the game. If the game slows down, simply refreshing the page should bring it back to normal speed for a period of time. It gets annoying after a while, but it is a lot more bearable than 4 fps.
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