With DS I had contractions every 5/7 minutes for 14 hours (take my breath away contractions, very painful).
When I went into the hospital they checked me out, the contractions were registering on the machine thingo, but my cervix was long and I was tightly shut. They told me I was experiencing prelabour.
They continued. Still extremely painful. At midnight that night (after now almost 24 hours of pain). I had had enough and went back to hospital with my prelabour. They were cranky with me for wasting their time. Checked me, and DS's head was right there, ready to be pushed out.

According to them it was still prelabour, and they counted my actual labour as extrememly short (something like 2 hours).
So yes prelabour certainly did shorten my actual labour.

But I think I had a shy schincter, I was extremely anxious about going to hospital (terrible first birth). I know youre an ina may fan, so I don't need to elaborate