It seems to be a balance issue. All I can say is that the rate you see now is what is intended by the formulas. It can be argued that it is going down too fast (this is a balance issue).
My well trained char with carrying over just 30% of his full capacity can run for 5 minutes, and then has to sit a minute. At full capacity I guess you will have to sit more often, but doesn't that make sense?
However, an untrained char carrying 25 apples showed a decrease 6 times faster. Even though resting rate is the same, I can understand that walking large distances will take a long while.