The 40 is a weird measure for sure. People get their best times coming low out of a sprinter's start, slowly rising and hauling ***. Not a typical football play.
The 40 was invented by Paul Brown (ancient history NFL icon) to measure punt coverage ability. Usage has become overblown and not thoughtfully applied.
Most football plays don't cover 40 years. Of course, DB and WR pure speed is important, but here we are talking about LBs. For a guy with reasonable speed, decision making, angle selection and just old fashioned playing fast can easily offset a tenth or 2 on a 40----especially when the issue is usually how fast are you in a 10.
More Flagg and Brooks!