Apply the registry patch to get Nero to show the exact burning speed. It's as a sticky in the Nero/InCD forum. This will help to indicate where the problem is.
But 9-10mins is not that slow so no real problem, maybe the media isn't actually burning at much more than 6x. My LG 4163B won't burn 8x Ritek G05 at more than 6x as it thinks that's best for quality (or all it's fit for).
What media are you using? Get the Media ID with Cd-DVD Speed.
You must however regularly defrag your hard drive as this is a major cause of buffer fluctuation.