DMA is probably not enabled yet. Even if you select the option "DMA if available" Windows often refuses to use it. The best way to enable DMA again is to delete the IDE channel your drive is attached to, reboot and let Windows redetect the hardware. That usually fixes the problem. Also, don't forget to check the DMA properties of your hard disks since they also need to be working in DMA mode.
Your question is answered in the FAQ under:
Q: My Plextor drive is performing very slowly. When burning it's continuously going into BURN-Proof mode.
A: The problem is that the drive is using the PIO Mode instead of UDMA mode. DMA must be enabled and working properly in order to achieve a smooth operation. Check this thread for info on how to enable DMA in Windows XP, 2000, Me and 9x.
Ps. Your Plextor DVD recorder is limited to 2x reading for DVD-Video discs to prevent noise and vibration. You have to enable SpeedRead so that the drive will rip your DVD discs at full speed. This is also covered in the FAQ:
Q: My Plextor DVD recorder only reads DVD-Video discs at 2x! Can't I make it go any faster than that?
A: Yes, you can 'unlock' the drive by making sure the drive is empty and closed. Then press and hold the drive's eject button for about three second until the drive's LED flashes green three times. Release the eject button so that the drive's tray will open. Now insert a DVD-Video disc and voila the drive reads faster. When you eject the disc or when you cut the drive's power, the PX-708A / PX-712A / PX-716A will be reset to 2x reading again. Update: As of PlexTools Professional version 2.14 the Plextor DVD recorders can be unlocked via the "SpeedRead CD/DVD" option.