I think I know what you mean. With high quality media like TY and Ritek(48X certified) there is usually no problems. But with some other brands like prodisc and moser baer (which is set to 48X recording in firmware) it often slows down to 24X and stays at 24X the rest of the disc. Disc with tiny scratches, dirt or dust may also cause this behaviour.
But in many cases this drive seems to give very low c1/c2 error numbers which is a good thing.
And it writes my old crappy postech discs at 40X with very low c1 ( <0,02 ) error rates. Finally I may use the rest of those discs.