First, a few questions.
What media are you using?
What software are you burning with?
What hard drives host the source data? (Size, RPM, interface type).
You may get a few more informed replies than mine, but here's what I know about this.
The BenQ has a unique feature that checks what it's burning, and adjusts things, including speed, so as to keep the burn as optimal as possible for quality, not speed.
This brings the media into question. This has a considerable impact on this feature. The better media can allow the BenQ to keep burning fast, though it will seem to "pause" occasionally to double check how the burn is going. If the media is marginal, the BenQ will be attempting to keep the burn quality up as required. Other drives that don't do this simply continue on to produce a marginal or failed result.
Now, Nero shows the drive's buffer fluctuation - discussed a few times here, but most often NOT below 85%. A dip to 50% might actually mean that you're starved - it might not actually show a drop to zero if it has occured - Nero doesn't - at least not the drive's internal buffer.
This is why I ask about the source hard drives. If they're fast enough to supply 15-20Mbytes per second, you can handle an 8 or even 12x burn, but unless they can reach sustained 21Mbytes per second +, you could be starving the drive, causing buffer under-run stahls.