If I mill the night before I brew, I can be done in a little over 3hrs (including clean-up).
What!?!
How is that possible?
Let's assume, as you said, you mill your grains the night before. Furthermore, you have all your equipment all set up and ready to go. Let's even assume you've already measured out your strike water.
Brew day starts. You light the burner and start heating your strike water. Say you've got a great burner and you're up to 168 F (or whatever your dough-in temp is) in just 20 minutes. So you dump into your mash tun and stir in the grains until you hit your precise mashing temperature (another 5 minutes).
You mash for 60 minutes. You vorlauf (1 minute) and drain first runnings (2 minutes). You dump in your sparge water (which you've already heated up, so no additional time) and give it 10 more minutes to "mash out". Vorlauf (1 minute) and drain (2 minutes) again, and fire up the burner for the boil.
Say it takes 10 more minutes to get your 6.5 gallons from 170 F up to 212 F (boiling). You get a good hot break within 2 minutes of boiling, then throw in your bittering hops and start the timer. You now boil for 60 minutes.
Flame out and immediately start up your immersion chiller. Say it takes 15 minutes to get down to 65 F (which would be amazingly fast, IMO). You then drain into your fermenter (3 minutes), aerate (2 minutes), move it into the fermentation chamber and seal it up (2 minutes).
Add all that up, and that brings us to 3 hours and 15 minutes, and we haven't even cleaned up yet. It takes me at least another hour to scoop the grains out of my mash tun and give it a good rinse, mix up some PBW and scrub my boil kettle, hop screen, and HLT, put away my equipment and update my notes.
How on earth can you do a full all-grain brew in 3-ish hours?