Hey,
I have had similar issues.
The version of the OS should hardly have any influence on your FPS in my humble opinion. I personally saw a great improvement when I switched to a Haswell processor last summer, but hardware upgrade should not really be a concern for a Rogue like…
One thing I noticed had the most influence on the FPS before changing my Mac was the size of the Terminal’s window (I think it is called “view port” in the option menu). You might have enlarged it be able to have a broader viewing distance. In my experience however, extending the view port has had dramatic influence on FPS. So keep it small!
Another thing to do is disable the tiles. The game runs faster with the native ASCII graphics… I could notice this even on my new hardware (at least last time I had a build that would work on computer): large view port and ASCII graphics run super well. If I turn on tiles, it is just like being back to the old days (i.e. prior to my hardware upgrade…).
Hope that helps…