I disagree.
I can get 350% framerate out of AdvMame.99 on an AMD 750 MHz athlon
Key word: ATHLON. The OP is using a K6-2, which has notoriously poor floating point performance.
Clockspeed doesn't matter here. While you're only 300MHz faster, the performance of true 686 technology with the Athlon core far outweighs what an old K6 (586) can push out, especially when it comes to float calculations. Not to mention your internal bus is several times faster than his, which alters performance for the better yet again.
My suggestion is to change emulators. RAINE is a fantastic emulator, and much faster than MAME, albeit with a smaller library of supported titles. If it plays the titles you like, try using it instead on older hardware.