In Double your productivity without more work or stress, the author makes the argument that the way to increase overall velocity is to decrease the amount of low-velocity periods of work. That’s because high average velocity (e.g. being “more productive”) is bounded by low-velocity activities over time.
For example, say you want to average 60 mph over a 60 mile journey. If the first 30 minutes you go 30 mph, it’s not possible to average 60 mph by going faster over the second half of the trip.
While working faster is a multiplier on the high-velocity work, the best bet is not to go faster but to eliminate low-velocity activity like email, meetings, scheduling, etc. that drag down your average velocity.