Running PS2 games

I know this has probably been asked 1000x times already. I've seen videos of people playing things like GT3 with very little slow down running 1x or 1.5x resolution. I've tried different settings but still get massive slow down whenever the grandstand on high speed ring comes into view. There's other games I'm having issues on but using Gran Turismo as a prime example. Edit: I'm using Aether SX/2