Two days ago. If you go to practice mode and test a hail Mary vs man coverage and just let them run all the way downfield you'll see the speed differences really looks good with threshold between 0-20. Looks a lot more realistic.
Edit : sorry read your comment wrong. I switched back to slow about. 4-5 days ago. I had it at normal cuz I do like Fast gamespeed but I also like Slow, here's why, on slow the animations and pass coverage is more crisp. It's more casual, as Fast does look like real NFL when you examine the player's run motions and how quick they get down field, the ball trajectory is pretty off on fast, the ball trajectory is pretty spot on when using slow, and at times some games have that slower feel, I think the fast feel is more for college football. For example these past wild card games all had a slower feel to them in my opinion.
Slow game speed also helps to get a good vision of the speed differences in players. It balances a low threshold setting.