Slower burns do not equal better quality.
Sometimes slower is better, and sometimes faster is better.
Often, the best quality is achieved at the media's rated speed, but not always.
Usually the drive will not burn at its highest quality when burning at the drive's highest possible speed (e.g. 16x DVD or 48x CD on the NEC 3500), because the disc will vibrate more.
From some of my burns I can see that the NEC 3500 drive burns a disc with higher quality when the speed increases in the next zone (Z-CLV), and 6x being better than 4x is quite common.