As pointed out by the developers here, MAME 0.193 seems to have issues in BGFX - at least in the Windows build. Although I did not encounter those in the official Mac build, it seems to me that the nothrottle command does not seem to work in BGFX because all games run at exactly 100% in BGFX.
Important Update: I moved to MacOS High Sierra due to Spectre and the Meltdown. Here I see a consistent decrease of performance by up to -15% in the games I had tested before. This seems to be consistent with predictions in the tech community. This observation currently only applies to OpenGL, since BGFX for some reason does not execute the -nothrottle option in MAME 0.193.
Before that, under MacOS Sierra, I had noticed quite significant performance gains under OpenGL/CRT-GEOM Deluxe of 10-20% in nothrottle mode.
And if you want to have an impressive overview of what happened in 2017 on the MAME side, this is a must read: https://mamedev.emulab.it/haze/2017-new-focus/
A very impressive summary and many nice screenshots. So that you know what you should test on your newer MAME systems.