I followed your advice and now the content of .cdrdao is:
but in spite of that, the writing speed is 40x (maximum).
I think all that could be due to the Writer options (or properties?) that are: JustLink (protection against buffer underrun) and JustSpeed (how to disable JustSpeed, if necessary?). Maybe such options pass over the write speed set in .cdrdao.
What do you think about that?