You ‘re right, I found this in the FAQ of
CloneCD: “Back in the early 90's the Redbook standard recommended to burn music CDs at 1x speed. However, back then the writers were only capable of burning 1x/2x and had no burn proof functions at all. The firmware of modern drives doesn't allow such slow speeds anymore. 4x is the least you can expect from your drives. Our software cannot override these firmware settings”.
I’ve tried with Memorex DVD+R DL 2.4x maximun write speed and the write speed was 2.4, How can it be if the minimum speed allow for the firmware is 4x???