crossg: I have all default settings. Haven't changed anything with WOPC yet. I believe it is UDMA 2. I remember it is the same as the PX-712A. But I will check again. I don't think it is the power supply because I didn't have problem with burning speed when I had two PX-712A connected (before I replaced one with DW1640). But I will try removing all the harddrives and DVDRW.
Hawseman: I don't think the defrag will help. Because the same thing happens when let CD-DVD Speed create data disc (not burning an image file from harddisk).
Quakester2000: Actually, I also have another harddrive connected to the on-board Promise controller. My power supply is 400W. So 3 harddrives, 2 DVDRWs. Does that sound good? Anyway, I will try removing them all. Again, I didn't have burning speed problem when I had two PX-712A drives connected.
Quikee: It's set to Accuracy.