LDW401S@411S speed problems

I bought this drive just before Christmas and upgraded it to 411S right away. Because DMA hadn’t been activated when i installed it, i had some problems with the first DVDs i burned using Nero 6.0. Computer got very “laggy” while burning, and it seemd like the HD’s couldn’t deliver data fast enough all the time. Burning CD’s was a bit slow too. Now DMA is activated, and burning CD’s works just fine. But burning DVD’s is still very slow.
I’ve just burned 2 PS2 DVD games on Samsung DVD+R using RecordNow MAX 4,5, and one using Nero 6.3, and it’s even slower than when DMA wasn’t activated. The biggest image, about 3GB, took about 3 hours to burn. Something is seriously wrong, but i can’t figure out what.

I’m running Win2k on a P3 500MHz computer with 512MB RAM, and burning from SCSI discs.
I really need a reebot again, but i don’t figure that’s the problem.
Is the computer just too slow?