Everyone pretty much knows what I think of him right now but that aside, I think there's an issue with team mates overestimating his speed. I've noticed more than once during the past few games that the through ball passes are often reached by the defender faster than Tello since they are further up the pitch. I don't think he's lost any pace but I do think that he may be doing something in training which he isn't reproducing in match settings which is rather weird since he'd be playing against Alba,Adriano,Alves and Montoya on the flanks in training and they are all rather quick.
Anyway, he actually does put himself in some good positions from time to time. Now if only he could finish...