Check out the capx, and try to test it to see that it lags. Sorry if i am not giving much information (ask me what you want). I think the snow makes the game lag, but im not shure...
Download Capx folder: dropbox.com/s/wxvs061wn8cke79/Why%20does%20this%20lag.rar
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It doesn't lag at all here, a solid 60 FPS in Chrome.
If you display the object count, it's constantly going up, which will gradually slow down the game more and more. Make sure you destroy objects that are not needed.
Ok, then it seems its just my pc doing something wierd... thanks :)