We had a firing stick a bunch of years ago for close to the same thing. Driver worked at 1 speed alone, but when he was with a supe, he always made his numbers. He was typically 1.5 over and scratched with a supe. Fired for not being able to work without supervision and it stuck.
The company did their part by documenting it very well. They did 3 or 4 rides and established a pretty solid pattern. The driver was working his own bid route.
Sounds like the OP is describing a similar situation. Over a 9 hour planned day, 15 vs 13 sporh is about an hour, maybe a little more.