I don't think this slider, Fatigue Effect, has anything to do with how fast the players get tired. I think it's a common misconception just like a lot of other sliders.
The players' fatigue rate is based on their stamina attribute. As the player gets tired, their play becomes sloppier because their attributes start taking hits.
By cranking it all the way up, I don't notice any shorter line shifts (Auto), but I do notice sloppier gameplay which I like. If you decrease the Fatigue Recover slider, it takes them a little longer to get back on the ice which results in more ice time for 3rd and 4th lines.
While most people have been turning down Control, Passing, and Shooting sliders, I personally have been maxing out the Fatigue Effect slider (and decreasing the Recovery) and have been liking the results so far.
I have also been messing around with the Attributes Effect slider and have been noticing interesting observations. At Default 3, there seems to be very little separation between players (EA rosters are too darn close to each other, just like the teams, every rating is between 87-90 it seems, lol). This also seems to make it harder to score.
Cranking it up to 5 or 6 makes it feel a little too arcadey for me overall, because all the star players become superstars etc. I do like seeing my Getzlafs and Perrys do the majority of scoring.
I'm wondering if 4 might be the sweet spot..