Harry Kuhman pointed me to the dma-cheat. A new test made clear that my speed problem was indeed caused by dma being off. 3.7 had it on by default, 4.0.2 had not. Therefore, my new suggestion is:
With Knoppix 4.0.2, at the boot prompt type "knoppix dma" and try burning again. Please post the result, I'm very interested in it.

More Good Luck
Dirk