Use my repro mission, or place an AI unit in the editor, give it 4 waypoints spread around 400m in a circle and watch how the unit comes to a crawl after roughly 40 seconds in.
Additionally you could add this into the units init:
nul = [this] spawn {
_fatigueunit = _this select 0;
while {alive _fatigueunit} do {
hintsilent format ["Fatigue: %1\nSpeed: %2 km/h\nTime running: %3",getfatigue _fatigueunit,round speed _fatigueunit,time];
sleep 0.02}
};