Stretch the limits of what you think is possible at least once a day. Crush yourself by setting aside fifteen minutes in which you will complete a 30-minute task.
You do this with an intensity of focus and a viciousness of work that you cannot sustain for a long period of time.
But by training your mind your intensity and speed of work will increase over the months and years.
Practice getting faster and focusing harder by setting hard deadlines that are very difficult to meet. Then voraciously attack the task.