Look at the name of the class: FIRST class. That means F goes first! Boarding coach first makes no sense on a two-class plane.

Yes, boarding coach first would speed up the boarding process (in general, boarding from rear to front). If UA is really interesting in reducing load time, they would rip out the F seats and have single-class service.

Having a first class means F passengers go first (check-in priority, first to board, first to deplane, etc.)