48x = approx 3mins
24x = approx 3.5mins
16x = approx 5 mins.
If your UDMA is not the issue (aka you have UDMA if available & the burner is using UDMA moda (or even multi-word dma2) this should be okay.
Please download (if you don't have it already) Nero cdspeed. Please benchmark a DVD if you have one available.
Also, nero has an infotool which can generate a log.
Please post your nero infotool log between "[ Code ]" & " [ /Code ]" tags (minus the spaces), important remove your serial if possible.
Unfortunately the problem you are experiencing can be caused by many things .. conflicting programs, aspi layer corruption, EZCD, ide drivers corrupted/ not working, hardware faults, cable faults & etc, etc.
The nero infotool log will help us fast-track alot of the usual questions.