LG GSA-4163B not burning at specified speed



I have a LG GSA-4163B and a NEC ND-1100A dvdwriter.
LG GSA-4163B - Master drive
NEC ND-1100A - Slave drive
And I am using Nero 6 Enterprise Edition ver
I am using DELL 2350 Pentium 4 2.0 Ghz, 512MB Ram
Running on Windows Xp Home Edition SP1
I have checked my DMA status, this is what i get:
LG GSA-4163B - Ultra DMA Mode 5
NEC ND-1100A - Ultra DMA Mode 2

Everytime I select LG GSA-4163B to burn at either 24x, 32x or 40x, it only gives me a burning speed of 16x max.
I have tried switching LG GSA-4163B to slave and the NEC one to Master drive and vice versa.
I have upgraded the firmware of both dvd writers.
I tried TDK Gold CD-R and Samsung 52x CD-R, BUT i still only get a Max CD Burning speed of 16x which takes about 5 minutes 20 seconds to burn a 700MB CD.
I dont understand why, Can anyone help me??