I've seen the all-at-once boarding on my last few flights and it does seem to work faster. I'm not exactly sure why, but if it gets the flight out earlier, I'm for it.

Last week, my flights arriving in (#199) and departing (#200) LAS didn't use the rear door. The rear door used to open fairly quickly after arrival for de-boarding and boarding was done through both doors unless it was a very light flight. I'm not sure if there's a new process or if there was a reason my specific flights were different this time.