Colston didn't have the best 40 time, but he had one of the best 20 yard splits. That's why the Saints took a chance on him and it paid off. Receivers run more routes under 20 yards than they do over 20 yards.
If a receiver has a good 20 yard time and a good shuttle time, then they have the speed and quickness needed to get open for most of the routes they will run.
The more precisely they run routes, the more success they will have. By the time a receiver reaches the NFL, it's easier to improve route running precision, than it is to try to get faster and quicker.