i always could choose 16x with my NEC burner . the actual speed was a little over 12x though.
now since i got a new mainboard you can only choose up to 4x speed so it takes maybe 10-12 minutes to burn (not sure exactly) before it always was 7-8 min if i remember correctly.
what can be wrong ?
my second burner is a benq 1655 and that one can burn 16x no problem.