The speed of electrical impulses is constant. What is not constant is the path that these impulses travel.
1 - Your brain has to determine the correct route for each finger command.
2 - Your brain has to determine the shortest route to send commands.
3 - Some of the routes must be improved for the increased electrical traffic. This means your body must grow your nerve pathways thicker to handle the increased load.
The path for increased speed is always slow. Why? Slower means accurate. Therefore, your brain has an easier time figuring out the shortest correct route to send commands. Your body responds by improving your neural highways.
It take 120 days for your body to renew itself. Therefore, you should track your improvements in these cycles.
Many players just don't get that its requires a physiological change (along with mental) that brings improvement. Even though your mental capability may be there, your body has to catch up. Ramming 10 hours of practice a day will bring improvement. However, you do not have to practice that much to trigger the physiological changes that bring about the speed improvements you desire. There is a reason why Tom advocates practicing a maximum of 3 hours a day.