Having a Master's will not decrease the amount of time it takes you to get a PhD, per se. However, if in your previous lab experience you learned skills that will help you in your PhD lab, then you might get through a little faster simply because you don't have to spend as much time up front mastering those skills. But even that is not likely to make a huge difference. Some people get their MS in a lab and then continue in that same lab for their PhD. In that case, if you count the PhD time as the time between obtaining the MS and obataining the PhD, then you could say that the MS expedited the PhD. However, this isn't a true shortening of the PhD time, because the time spent in research for the MS also applied directly to and augmented the dissertation research covered in the "official" PhD years.
I pretty much agree with CielloStelato's points.