After playing a few more games with it, I really like how "fast" speeds up the APF passing game.
The effect on the running game seems to be entirely negligible, but I definitely feel like the OL/DL interactions play out more realistically on "fast."
Here is the personnel I've been testing:
With that matchup, I have time to complete my drop, then the ball basically has to come out within a second of completing the drop or I am toast.
On defense, I was running:
Vanden Bosch was getting double-teamed a lot by the left guard Mcintyre, but it just freed up Suh to go 1-on-1 with a generic center and he was eating that generic alive.
The reason the pass rush seems like it gets there faster on "fast" is because the animations that go with the OL/DL interaction don't take so long to play out.
On "normal" speed, there's no real consistent pressure from legend DLmen because a lot of the "interaction" animations simply take too long to play out compared to how fast the WRs run their routes or how quickly the QB drops back.