Don't fool yourself. Emulating Game Boy Advance hardware is no small feet. Seeing games like Advance Wars running as well as it does on the N900 is amazing as I've run GBA emulators on S60 3rd edition devices and watched it absolutely crawl.

As a rule of thumb ( if I remember correctly ) to properly emulator one type of CPU on another type, the emulating CPU should be at least four times as fast as the original CPU's speed.

That's not to say that faster CPU's can't be emulated on the N900, it's just that you'll see a more noticeable performance hit. Just search You Tube for PSX Emulators running on Maemo to see this in action.

Having said all that, of course the standard suggestions apply to speed up VGBA performance: Close as many background apps and desktop widgets as you can to free up CPU cycles.