I think Ubuntu has way too much stuff you don't need for an arcade cab, that might slow it down.
I use Lubuntu 14.04 now which is a much lighter weight system.
My first mainboard was an athlon Xp 2100, 750 Mb mem and an Ati 9250, even then it worked fine.
You can try using monitoring software, like top, to see what's hogging the system.
The best option would be switching to Lubuntu 14.04