The newer version of the game (updated in 2014) can react sluggishly on some Windows computers. We do offer a few options for this situation.
One, like leecie suggested, is to play the game during the day or have it set to daytime whenever you play, and to close the game and reopen it if the fog causes it to run sluggishly.
The other option would be for you to write into support, and I can provide an older version that should run better for you.
If you'd prefer to get the older version, simply follow the link in my signature, and mention that you are Emmabunny from the forums.