Burning speed is determined by information stored in the firmware, and no burning software can change these information.
However, with currently available media (mostly 16x rated) it's not a good idea to burn slower than 8x. Usually with 16x media best results can be obtained burning @12x or @8x, whereas @4x results can be really poor.
The only way to burn @1x is using an older hardware and low speed rated media (both really difficult to find).
Just a curiosity: why do you want to burn @1x?