I have a 16x DVD writer and have found that the 2 major things that change the burn speed are:
1 - The program you're using, if the program is not up to date or for some reason doesn't like the disks it will refuse to burn at the highest speed and will reduce the burn speed accordingly.
2 - The disks you use make the greatest difference, I've had 3 makes of disks, all rated 16x but only one would burn at 16x the others would only work at 8x. Now whether it was the DVD writer or the program that dropped the speed I don't know.
Are you sure that the disk you are using is a DVDR disk and not a DVDRW because these burn much more slowly. If you are sure then buy another make of disk in the first instance and try that and see what happens. Incidently the same thing can happen with CDs if the writer or program doesn't like a particular make.