The real lesson should be to not worry about 40 times and metrics and just go with production and what you see on the field.
Teams should make optimum usage of all available information.
There is some information in forty times but fans/media and some teams do over-focus on forty times.
Some players lack of speed does not clearly show up on college game footage.
Darian Thompson, currently a Safety for the Cowboys, had good "game tape" from college and was drafted in the 3rd round.
He had a slow forty time and lack of speed/range has proven to be his limitation in the NFL.
Forty times for true workhorse RBs does not seem to be an indicator of success for the most part; although, slower RBs might not be a fit in all offensive schemes.