@Flandry

Read that the new versions of mame 0.138,0.139 for example are having problems with under-powered pc's and are showing slow downs and sound stuttering and people are advised to return to older versions to experience smoother gamplay on both graphics and sound syncing on older roms space harrier, r-type,nemesis(gradius) etc. i haven't read any of your posts recently so i don't know how busy you are. Is there a chance you could possibly look into doing a quick port of MAME build 0.37b5 from the wiz or pandora versions just to see if they can maybe run some of the trickier roms better?

Ran my GP2X WIZ at 533mhz on mame4all 2.5 (build 0.37b5) tweaked a few settings and got outrun running at 76% speed at 18fps on frameskip 7, very playable. so surely the N900 can run this emulator at similar if not better performances