There are a number of possible causes for your framerate dropping while you play the game.
The top priority is to Check for Overheating of your CPU or video card.
Pause any Background Downloads that may have started since you began playing.
If Path of Exile - or other software running on your computer - fails to adequately release RAM as it runs then your computer can run out of memory, which will cause significant performance problems.
Create a Thread in Technical Support if you would like help looking into this.