LG 4163 burners, for example, produced a lot of jitter, depending on the media (and firmware) used, when burning at 16x. I just mention the LG 4163 because it was a big deal, at the time, for a 16x p-cav burner (pretty quick burns). The tradeoff was high jitter levels, initially. This is one reason why I never get too excited about faster and faster burn speeds. If the burn quality is still good at 18x and 20x, then fine. But if the burn quality sucks, then who cares how fast people with 20x burners are burning?
Also, many burners seem (according to scans) to produce better burns on Verbatim +R DL media at 4x rather than 8x or higher.
It's possible to burn too fast and too slow depending on the burner and media used.