The are a couple of things you could try.

From within the software, go to you VIDEO settings and see if you can disable all the extras such as; shadows, ground detail, clouds, etcs. Now lower the resolution and color quality, say 640x480 instead of 1024x768.

Play with the settings in the sofware until you get it to run at a good speed. Now you can enable what you disabled one at a time, until it starts to slow down again.