My opinion of how each feels.
Very Fast - Arcade like. Fun for non realistic football fans.
Fast - Feels and looks close to real NFL speed feel. NFL is played at a high speed.
Normal - feels at average speed.
Slow - simulation style, enjoy the slower crisp animations.
Very Slow - Sluggish. Feels like running in mud but can produce beautiful animations and interactions. Footplanting / movement feels sluggish but looks nice.