Phoenix are awful and boring. It got worse recently with all the pointless round-a-bouts they built. There is a spot just before Kingman where you can get caught in a 2 hour delay to turn left onto the freeway going up the hill to Kingman.

I take the Flagstaff route from St George and it is 30 minutes longer. It does vary from your route as we drive over Jacob's Lake area. I'll even add another 30 minutes and drive thru Sedona for a meal or to take in a hike. The only stretch that is tiresome is around Cameron/Tuba City area.

Vegas to Phoenix you should always consider that you are going to put massive wear on your suspension and tires. Lots of the roads in Costa Rica are better.