I had a similar problem, except it was a double whammy. On my Promise Ultra133 TX2 IDE controller my DW1620 could only burn at 3X, and firmware flashing took several minutes. On onboard IDE (VIA KT266A chipset, VT8233A south bridge), it could not burn at all, nor could it be flashed. Finally I figured out that it was because I was using the VIA IDE miniport driver which is now obsolete (though you can't tell by reading VIA's page)... switching to the Microsoft VIA IDE driver fixed the problem.
That makes it sound easy... in reality there was much anguish and futile tech-support calling and web searching involved before I discovered the solution. The BenQ techs were utterly clueless ("You can burn a disc on 20 minutes? That's fast!"). I came close to returning my DW1620. Web searching eventually showed me why my Promise card wasn't right for the job. But I had to figure out the driver switch on my own. Hopefully this thread will show up in somebody else's search and save them some anguish.