I used to live in western Arizona a few years ago. The drive time from Flagstaff (where you turn off I-40) to where I lived is comparable to the drive time from Flagstaff to Phoenix. Driving from the northeast down I-81, I stopped overnight in Knoxville. My drive segments and overnight stays on I-40 were:

Knoxville - Little Rock
Little Rock to Amarillo
Amarillo to Albuquerque
Albuquerque to (Phoenix) Arizona

I traveled with my dog, so the trip/segments were fast, but not to fast. (Numerous "rest" stops, both for the dog and me!)