Statistics are all in how they’re presented. In the last 30 years there have been about 4500 ERJ and CRJ airliners built. That has certainly slowed down the average airplane speed. There’s no reason to operate a half full 737 to the middle of no where when when you have RJs that can do the route a bit slower but much cheaper.
Cost Index in the 747 goes from 0-600. I’ve seen days when the release says 45 and days when it says 600. On those go fast days days it’s all great until ATC says we're following an RJ doing .74 so slow to .73.