I'd be very suprised if what you said was at all correct, firstly how are you measuring the speed?
Any burner rated at a speed does not start burning at that speed the LG for example probably starts at about 12 and gradually works up to over x40 giving an average somewhere around x38 this is with perfect media which is hard to find.
Get yourself CD Speed 1.01 which is a Nero tool it will show you what is happening.
The LG 40/12/40 is limited at lower write speeds you can read a review at www.cdrlabs.com however it is rated as a very reliable and fast burner using P-Cav.
Why do you want to control the burn speed in the first place?