This is breaking many missions which depended on AI movement. Not to mention that it's extremely easy to take out fatigued AI units because they simply won't take a rest until they're not fatigued because they don't know it.
At least integrate the fatigue into the AI FSMs so they'll rest when fatigued.
It would probably be more authentic to make the slowdown happen once fatigue reaches 0.8, slowly increasing the speed reduction until fatigue hits 1.
When fatigue is at 1 for at least 15 seconds then it could be reasonable to further reduce the units speed, same goes for the player.