Some more testing.
Software renderer (default)
Mac 10.9.5: Works great, snappy performance, no glitches.
Mac >= 10.10: Starts showing graphical glitches (closing a window by clicking a button leaves a black hole at the position of the button until the window disappear, which takes a while…) as well as the overall performance has gone down considerably, everything just feels slow.
OpenGL renderer.
Mac 10.9.5: incredibly slow, basically unusable, we have some hover effects that when are being rendered completely locks the entire OS. (spinning ball of death until mouse stops moving)
Mac 10.10 / 10.11: Slower but still usable (well, still incredibly slow compared to software rendering in 10.9.5), all glitches are gone.
Mac 10.12: Performance is about the same (compared to software renderer in 10.12), all glitches are gone.