Hi Germanbear!

My suggestion: Try to find a knoppix 3.7, boot that and try another burn with k3b. I observed weird speed penalties lately, also with burning (4x max. speed) with a faster system than yours. Right now I suspect unionfs to be the culprit. Knoppix 3.7 didn't use unionfs yet, so if I'm right, you get what you need

Alternatively, you could use a different life CD/DVD that does not use unionfs. But not Kanotix, at least the latest one seems to have the same problem as the actual knoppix.

Good Luck!
Dirk