As it stands now, if your carry weight is 400 pounds, you can carry 399.9 pounds and move just as fast as if you were naked and carrying nothing...but pick a single tiny flower, and suddenly you can only walk slowly.

Is there a mod that alters this? Maybe so that encumbrance penalties start coming in gradually once you hit around 50%-75% of your carry limit, and running only becomes impossible around 125% or so?

This would seem to be considerably more realistic than what we have now, although the vanilla Skyrim system is better than Oblivion, where being overencumbered meant you were immobilized.