In 2000, the average home computer ran on a Pentium III or an AMD K6-2. Emulation was computationally expensive. Later versions of MAME (0.100, 0.200, etc.) focused on accuracy over speed . This means modern MAME simulates the actual circuitry of arcade boards down to the electron level. That requires a 3GHz processor.
In the emulation world, ROM dumps change. A game dumped in 1998 might have had bad audio samples. A re-dump in 2005 might correct a graphics glitch. If you try to use a modern ROM (from MAME 0.250) inside the MAME 2000 core, it will fail. The CRC (Cyclic Redundancy Check) hashes won’t match. MAME 2000 Reference Set - MAME 0.37b5 ROMs and ...