Use Nero DVD Speed and AnyDVD to test the transfer speed of a "pressed" DVD dual-layer movie. The NEC 3500A should start out at 3.1X and top out around 7.3X just before switching to the second layer of the DVD movie. Here, the speed should slowly drop from 7.3X to 3.1X when ripping the second layer. This is the most accurate read test.
If you see similar performance, then the problem could be coming from the ripping software (remove/reload) or the DVD disc.
Copying an encrypted dual-layer DVD will always take longer than a single, unencryted DVD (from 7X to 16X). Ripping an average 2 hour encrypted DVD with ONLY the main title, Englich AC 3/6 audio, and NO compression will take between 12 and 15 min.