Nobody is answering because nobody has a perfect answer, really just anecdotal. It would take a long time to do a proper scientific study to confirm anything, but in my experience I would say fatigue does affect performance, and probably enough to be noticeable. Not drastic mind you, but not quite inconsequential. Ditto with injury chances. Lastly DHs seem to lose energy equally as much as SSs or any other position player as silly as that may seem.