bgfx_screen_chains hlsl
(You should check may also have to set bgfx_path to the bgfx directory if MAME cannot find the shaders.) Check with F11 if you still get 100% performance with BGFX and if . If not, adjust frame skipping skip with F8/F9.
Also, if you press the tilde (~) and then cursor up/down, you can adjust the shader settings. These are not saved at the moment though.[http://docs.mamedev.org/advanced/bgfx.html]