Before doing anything, I documented this event and I know it would be interesting to post in Pimp-My-Rig after resolution of the problem. To give you a baseline, my D630 is running the latest SP3 of Windows XP Pro up-to-date with the November 2008 patches and has the recently released A14 BIOS installed.
The first area I checked was if the DVD-RW was working in DMA mode. And this was where the problem was -- the drive was working in PIO mode, as seen from the screenshot below.
I then tried to unplug the drive and re-insert it on the bay but then the PIO mode was persistent. Even when the IDE driver itself was re-installed, it didn't solve anything.
Before doing a reboot, I tried looking for answers from the internet and this site had the exact answer. It involves modifications to the registry, so in replicating the same procedures in your computer be sure to have a backup.
[1] Open Registry Editor. Start --> Run.. --> regedit.
[2] Locate the key: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Class\{4D36E96A-E325-11CE-BFC1-08002BE10318}\0000
[3] Edit --> New --> DWORD Value. Add the entry "EnableUDMA66" and give it a value of "1".
[4] Quit the registry editor and restart the computer.
After reboot, the NEC 6500A DVD-RW was now working in Ultra DMA Mode 2 and burning speed was back to normal.
I hope this experience also resolves similar problems you encounter.