A scalable, energy efficient process for preparing cellulose nanofibers employs treating the cellulosic material with a first mechanical refiner with plates having a configuration of blades separated by grooves, and subsequently treating the material with a second mechanical refiner with plates having a configuration of blades separated by grooves different than the first refiner. The plate configurations and treatment operations are selected such that the first refiner produces a first specific edge loading (SEL) that is greater than the SEL of the second refiner, by as much as 2-50 fold. An exemplary high first SEL may be in the range of 1.5 to 8 J/m. Paper products made with about 2% to about 30% cellulose nanofibers having a length from about 0.2 mm to about 0.5 mm, preferably from 0.2 mm to about 0.4 mm have improved properties.