you don't have vsync enabled or anything similar. (I think its vsync... might be named something similar). This makes mame run the same fps as your monitor... Not as fast as the original game. So if robotron original ran at 50fps, then youve just jumped it 10fps (about 15% increase).
Next.
Robotron is very hard. It was a Zen or twitch game. It also sucks on larger then 19" monitors because you need to see everything at one shot.. Anything larger and your missing stuff in your perifial vision. I use the bezel for it just to decrease the size.
But play it 10 times in a row... and see if you can slip back into the zen of robotron. It helps if you say "OOoooohhhhmmmm" while playing.