Second labours tend to be faster because your cervix dilates and thins out at the same time. First time labours it effaces first and then dilates. Its like a muscle memory.
Also the pelvic floor muscles have been there before, the birth canal has stretched out before....the body on general thinks aha childbirth, done this before! Even ladies having a VBAC who have laboured before tend to have quicker progressing labours than their first time round
My own diagnosed labour first time round was 3 hours, including delivery of the placenta.... so goodness knows when I have another itll just fly out. Having said that...took them to break my waters for any progress to happen, I went from 4cm to fully in 20 mins and they didnt believe me when I said I needed to push. That really, REALLY hacked me right off. Like Im stood here doing it involuntary for the crack.