when burning from hard disk i can get full 48x, only because my media is 48x max and my burner 52x max. i'm sure it could do 52x fine. i have seen the slow burn problem before though.
sometime it works okay than it gives 'cannot close disc' errors. i think the problems could also be software related as Target mentions.
if you do a cd-to-cd copy you should try putting the drives on opposite cables. sometimes they have problems if on same cable.