Last time I messed around with MAME pretty much everything which was playable ran at full speed, even stuff like STV, Namco System 22, and Seattle based games. This was on my 2600K running at 4GHz, which isn't exactly the latest and greatest.
From what I understand the most demanding thing to emulate isn't the 3D hardware anyways, it's the high clock rate CPUs. Most of the latest 3D hardware supported by MAME is implemented with ASICs which have a relatively high level interface. This allows the emulation to be pretty highly optimized since the low-level details aren't visible to the game software anyways. CPUs are a whole different story though. There you are stuck emulating individual instructions which gets pretty hairy when the CPU you're emulating can do 200+ MIPS.
From what I understand the most demanding thing to emulate isn't the 3D hardware anyways, it's the high clock rate CPUs. Most of the latest 3D hardware supported by MAME is implemented with ASICs which have a relatively high level interface. This allows the emulation to be pretty highly optimized since the low-level details aren't visible to the game software anyways. CPUs are a whole different story though. There you are stuck emulating individual instructions which gets pretty hairy when the CPU you're emulating can do 200+ MIPS.