The only other obvious thing that will slow down the burning is the burner & the source (HD) being on the same IDE channel. But then you’d get fluctuations in the buffers so this is not too likely.
You could try another IDE cable, preferably 80wire, as this can sometimes affect things.
Any chance of posting a Nero log as this might help?
What is the media you’re using. The brand & media ID for the DVD media would be useful.
I’ve got some Ritek G05 DVD-Rs, rated at 8x & recognised by the firmware as such. However my LG burner will only ever reach 6x. For some reason the burner refuses to go faster, but nevertheless gives me a very good burn.