Apologies for refreshing this somewhat-dormant thread but it was a more streamlined choice than opening a separate thread and possibly re-duplicating the concerns of this series of posts.
To the topic at hand - those wishing to squeeze out a few more FPS in
FG, on their AMD-video rigs, be they of Mac, Windows or Wintosh flavor, might be interested in glancing over
this post under the relevant
CombatAce thread for
FlightGear tips/suggestions.
I am getting good results (i.e., slightly improved FPS) with the OpenAL driver replacement suggestion that's outlined there. Also recommended is to check over the very first post of that thread too for other tips regarding wholesome FPS in
FG, on AMD-vid rigs.
EDIT: the tip mentioned in this post may not be as helpful on Linux, considering the possibility that you are already perhaps running newer OpenGL drivers in your Linux-variant
FG installation. The tip will most likely be of greatest help on Windows systems and/or Wintosh hybridizations where Windows is installed on a Mac. Can't say anything regarding Mac OS X/macOS because I don't have
FG installed on the Mac OS side of my computer.
Good flying to you (with stable FPS of course).