It doesn't matter what the speed limit is. He's going to drive five miles slower.
And it doesn't matter to the guy in the next car either… he's going to drive seven miles faster.
It's not absolute, it's relative.
The person wearing the underachiever hat (it's temporary and he's a volunteer) will get a C+ no matter how difficult the course is. And the person who measures himself against the prevailing standard will find a way to get an A+, even if he has to wheedle or cut corners to get it.
When leading a team, it's tempting to slow things down for the people near the back of the pack. It doesn't matter, though. They'll just slow down more. They like it back there. In fact, if your goal is to get the tribe somewhere, it pays to speed up, not slow down. They'll catch up.