Run CD-DVD Speed 3.61 Create Data Disc tests and post the results here. Also include media information using DVD Identifier. Run Disk Quality Test tests on BenQ DW1620 and 3.61 CD-DVD Speed.
From Task Manager, close all programs and processes that use CPU power that you don't need right now. Set the CPU priority to high or real-time for CD-Speed and Nero Burning Rom (or any other recording software you are going to use.) Disconnect from internet and turn off virus-scan software and other things.
Also try to run defragmentation software. Re-partition one of the two HDDs which is not used for OS partition and let it have at least one partition for storing files for DVD recording and then use the files stored in that partition for tests and real backups.
Use only the best blank media for tests and when you are unsure. Try different speeds and then compare the results among them.