Is DMA on for both HD & burner? HD would probably be UDMA mode 5 & the burmer UDMA mode 2.
Also it's essential that the burner & HD are on different IDE channels.
It's possible that your spec (p3 800) is an issue but I would have thought you should get more than 2X. Try defragging your HD as this can make a substantial difference.
Note what's happening to the 2 buffers in Decrypter. Typical the device buffer would stay around 90%. If they keep dropping badly then fragmentation could be the cause.
If the TY's are genuine the minimum you should get is 8X. My Benq burns these at 16X.