My personal experience - Both of my babies have been posterior up until right before birth, but both labors have been fast dilation with a couple hours of pushing (6 hours total labor for #1 and 4 hrs for #2). I do not really feel back labor though. I think it has to do with how my pelvis is shaped. I get adjusted regularly throughout pregnancy and do tons of pelvic rocks also so that may help. Chiropractic has been shown to make for shorter easier labors.
Sending good labor vibes your way mama! save your energy and don't forget to eat and drink!!!!
Larissa, mom to all my girls (07/10), (05/02), (09/98) and another due 02/15!