I cannot imagine you wrote your answer on a PC/notebook that runs with less than 1 GHz. When it comes to speed, the MHz and GHz do count.
Your example (traffic light) is only valid when booting a system. A Ferrary can reach 100 km/h in less than 4 seconds. How many meters will you be able to cover in 4 seconds?
I do love my two Arduinos.
However, when it comes to real speed, the Arduino might not be the proper solution.