There are just too many MAMES out there and it gets confusing. :wacko: It would be great if just one MAME emulator could play all roms.
I have 4 different MAME emulators on my Pandora.
The reason for that is that MAME was never meant "for playing games", as it were. The stated goal of MAME is to emulate as many machines as possible as accurately as possible. This means that occasionally some feature will be reworked to be more accurate (or to incorporate additional systems) with an unfortunate cost of being less efficient. This is why we have multiple MAME versions: one version may play some games very well, but a later version implemented new features or changed things to be more accurate and can now emulate more games but is unfortunately slower. On a desktop this doesn't really matter: you've got the CPU power, just keep updated and you will have no problem playing old games as well as the new, but when we've got very limited CPU power like on the Pandora or any embedded system it's a delicate balance.
Other emulators have the stated goal of actually being able to play games, and for those there are optimizations and hacks that can make the emulation much faster at the expense of accuracy. The lost accuracy is usually not noticeable (things like memory management, or HLE, etc...) but such things go against the goal of MAME. There isn't anything we can (or really should) do.