I suspect he was already declining during the Sylvia fight, but he finished it so fast we didn't get a chance to even notice. Then he looked sluggish against Arlovski and wasn't moving anything like he used to, but again finished Arlovski before we could really notice. Brett Rogers showed that while he was still on another level from the average fighter, he just wasn't the same athlete, and then Werdum sealed the deal; didn't have the explosiveness to stop Werdum from locking his legs in the first place, never mind explode out once it was locked as he was known for doing. Granted Werdum is one of the best BJJ guys ever, but Fedor was no slouch and was more than capable of stopping him from getting the lock in the first place.
I'd say the last time he showed real explosiveness was against Choi.