I might be imagining things, but I thought the performance penalty was spelled out specifically at one point in time. Fresh and fairly fresh = no penalty, tiring = 10 percent penalty, tired = 30 percent and very tired = 50 percent.
So it would impact across all facets of play, but you'd see it more readily if the guy was typically a high distro player.
Assuming, of course, that I'm not imagining things from too many Peeps this weekend...