Pioneer A06 : very slow when copying CDR

I was trying to copy some CDR & DVDR last night and noticed an anomaly
with the Pioneer A06. I was copying to the "image burner" (Nero 5.5).

With DVDR as the source (burned with the same Pioneer in the morning),
the speed was about 3,500kB/s.

With CDR (also burned with the same Pioneer), the speed was hovering
between 500kB/s & 750kB/s.

My system is XP2500+ and I believe the setup is done properly (Pioneer
& HD on separate channels, UDMA turned on, etc etc).

What else could explain the low speed with CDR? A 10X speed would have
been the minimum I was expecting.