If it's the old socket 939 Athlon 64, the processor is too slow.
If it's an AM2 socket X2, then it's borderline, but should still be able to squeak by.
For Donkey Kong, I'm guessing HLSL was enabled and the prescale was set high.
Otherwise it should have had no problem running full speed.
For Tekken 3, Soul Calibur, etc., the processor is probably just barely enough and will require tweaking.
Having a 64-bit OS and 64-bit version of MAME helps.
Definitely can't have HLSL enabled at all on those games. No antivirus, nothing running in the background.
For Tekken 3, I remember ending up running one of the clones instead of the parent because it ran better.
There are also 2 completely different versions of Soul Edge. IIRC, I've only got one running full speed.
Dreamcast version is the best.
Using an old version of MAME might help also. My old setup uses somewhere around MAME 127.
The big thing to remember in MAME is that the GPU doesn't help much. The entire machine is being emulated by the processor, 3D rendering and all.