You can't burn slower than the slowest burning speed supported by your CD/DVD drive.
Even if you use another program that allows you to select a slower burning speed than your drive supports, the drive will just burn at its slowest supported burning speed instead of the selected speed.
I suggest you try other CD-R media or other burning drives.
Pioneer DVR-112L, LiteOn SHM-165P6S, Pioneer DVR-111L, BenQ DW1655, Plextor PX-760A
LiteOn LH-20A1P, Optiarc AD-7203A, Matshita UJ-840S, Sony RDR-HX910, Sony RDR-HX750