I suppose it being a Java emulator aswell as the complete lack of filters as the screens are smaller than GBA screens aswell as most phones having fairly limited sound quality help somewhat.

I never used filters on my Celeron 1.7Ghz so I would not know how they affect performance...but I can tell you that without filters and stretching (or at least not more than 2x or 3x) even a P3 (never had a P2 to try...a P1 lags a fair bit though and it's somewhat frustrating playing on it) can handle VBA just fine. You have to have a somewhat decent Video renderer though (for that time...a TNT2 does the job just fine). I was so surprised when I got my Celeron 1.7Ghz with 256MB RAM which was double the power I had before...and VBA lagged....turns out the SiS650 onboard graphics card was so bad it could not even handle games which did not even require a propper video accelerator aswell...as soon as I stuck a GeForce 5200FX in there though everything was fine.