While the 115 is pretty miserable, it the fact that at 10 at night it is still over 100 degrees that I find miserable!
As others have said the drive is easy. If you are staying in Phoenix west of the stack (I-17 and I-10) then leaving at 7 am won't be a big deal. But leaving at that time where you will have to go through that area will be heavier traffic. We do the drive all the time from the north valley (north of scottsdale) and it takes me and my mom about 6 hours depending on how many stops. My dad can do it in 5.5 easily but he drives ridiculously fast and doesn't support many stops. Quartzsite is my favorite place to stop and usually has the cheapest gas and the cleaner bathrooms. Between Blythe and Indio is no mans land. Take the 60 if you are comfortable with high speeds in curves with trucks. But the time of day taking the 10 all the way to the 91 should not make much difference. The 91 can get really busy and slow going. You will be hitting it just as traffic normally starts but there is never really a time when it is empty. Around the 55 is bad because it is a left exit, so stay in the right lanes. If you don't like traffic you can always get off after the 55 head south to Ball/Taft or Katella and head the rest of the way on streets. I prefer Taft/Ball (the name changes at some point) because the lights are better and less traffic.
The gas station at Chiriaco Summit is roughly halfway between. We call that stretch between Blythe and Indio "a whole lotta nothing."
