Try toggling between full / windowed

Or, try setting the hard GPU framesync setting to 1 if it's 0.

Or 0 if it's 1.

Looks like the problem was that I was looking at the frame rate in the graphical menu, without actually running any game. I followed your instructions without actually changing any settings, and now it works just fine, both in the menu and while running a game. This is what I did:

• Switched from windowed to full window. Framerate fell from ~40 FPS to ~15 (in the menu screen, without actually running any game)
• Switched back from full window to windowed. Framerate went back to ~40 FPS.
• Tried to open a game. Pressed ESC by mistake and the emulator closed.
• Ran the emulator again. Now the animated background in the menu runs very smooth (windowed).
• Tried switching to full screen. Still smooth at about ~50 FPS.
• Launched a game. Runs smooth at 60 FPS.

Looks like whatever it was, closing the emulator and opening it again reset something and it's now fine:

Now to find out how to take screenshots and how to record video from RetroArch.

Thanks! :-)

Ok, I'm able to reproduce the performance problem. It's the dual display.

When I'm at home my MacBook Pro is plugged to a 27" display. Most of the time I'm using both the laptop's display and the external one. However, when I need space on the desk I close the laptop and put it out of the way, using only the external display.

Using only one display (either the built-in or the external one) performance is perfect. No issues in this case.

It's only when I'm using both displays that performance drops, regardless of the settings.