In a recent podcast I referenced "can do" challenges such as Paint the walls in a 10' x 10' room in 6 minutes with a brush and roller. This video of the Proform Picasso shows the properties of the brush that Jack Pauhl used in that "stunt" - a stunt with no tricks, just consummate skills, efficiency and a great set of tools.
The Picasso brush is still available today.
He also trialled a Fox Paint brush from UK, designed by a former work colleague of mine. Imagine being able to load a 2.5" brush and Paint a dead straight line as far as you can reach, and still have enough paint left in the brush to Carry on another metre.
The only other brush I know of like that is a 3" wooster FTP, a Big ole brush by any standards. A 4" block brush might be able to match that, but it's another step Up in size and weight, so a tool that requires a lot of practice and a strong wrist.
So for a top tip, find a 2.5" brush for cutting in. Roll over the brushes line with a mini roller as close to the Edge as you can, then roll the wall using a 9-12-14-18" roller with a sleeve with the same like as TVE mini roller.
Happy pro #painting