Flash has always been sluggish on OSX, but the beta of Flash 10 promises to be faster, according to one of the developers blogs:
If you have followed GUIMark at all you will notice that this version of the player runs this benchmark substantially better on OSX than any previous Flash Player version. It should be up to 3 times faster. How will this affect you? Well, OSX device text rendering got a huge performance boost. If you use lots of device text you will see a big difference.