Are you playing the game via a USB drive or some other kind of external hard drive? That can cause performance bottlenecks that can really drive the framerate down. Try installing it on the local drive you're playing the game on.
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If you have a save from before you entered the desert, it might be an idea to go back to your room at the inn, interact with the journal there, and disable the game's UI effects for a while. Some of the desert ones can cause some machines to lag.
Other small things that might be relevant:
Make sure you're playing the most recent version of the game (v0.308)
Check your computer isn't in power-saving mode or anything else like that which could hamper performance
Playing the game windowed as opposed to full-screen can help slightly too.
Other than that... it might just be down to your computer's performance and specs, I'm afraid.