I think 40 time is a gauge, but sometimes doesn’t accurately describe speed. The difference between a decent and great start can be .1-.2seconds. In live play, DBs don’t really start until the WR starts and then the race is down to how fast do you really run instead of who gets out of the blocks faster.
Inverse seems to be true for pass rushers where quick start over OL is probably more an advantage than pure speed.
Just my $.02 worth…