The last part isn’t true, at least unless there has been a grave accident. Many pee on themselves during races, but most of the Number 2 is done at port a potties along the way with an assist from the team car or peloton to get back up to speed and get back into place. You might be surprised at how little someone might need #2 when all their calories are from highly digestible into energy food.

It’s a double whammy because every ounce of shit is one more ounce to drag across the finish line.

I think there's also some other physiological action that trigger defecation as a response during long duration high intensity exercise like marathons and bike races.

Edit: here

"It’s related to the fact that during periods of physical stress, the body shunts blood away from organs that are not necessarily critical at that moment,” Michael Dobson, D.O., a colon and rectal surgeon with Novant Health in Charlotte, North Carolina, tells Mental Floss.

https://www.mentalfloss.com/article/648541/why-marathon-runn...

The reduction in mass undoubtedly makes it easier to complete the remainder of the stage.

It might add some propulsion force if it is diarrhea