Well 90 minutes average is not too bad, but you should try for better. I start by naling the hinge side of the jamb to the framing, after I check that the framing on that side is plumb. Then do the latch side using shims.
Just to make you feel better, I just finished installing a prehung double front door that was 4" narrower than the original. Took a 9-hour day to get the old one out and the new one in and lockable. Then another 9 hours or so to do miscellaneous patching and filling.