I have the same problem. It actually started by the release of 1.2.4 update. But in 1.3, the game is unplayable on resolutions higher than 800x600 as it drops down to 5 fps while there's rain. Even at 800x600 it drops while there are a couple of things going in the screen. My specs are i3-2350M, Geforce 410m and 4GB of RAM.