Arcniz, here are some links to research into the concept of a Takeoff and Performance Monitoring System, which you discribe:
NASA TOPMS research
University of Saskatchewan thesis on TOPMS system
NASA TOPMS cockpit display development
As you can see, the concept of a Takeoff and Performance Monitoring System is being worked on by some groups. That's a good thing, because I think it's a great idea.
I also agree with RatherBeFlying, as long as the time vs speed parameters can be derived fairly easily, the stopwatch will work until a more sophisticated system can be created.
(edited to add a comment)