The key word is performance. The Leicester team is not better than ours on paper. But what they lack in quality they have more than compensated for in spirit and performances. The absolute lack of it is what's going to cost us this season.
I agree, we're performing far from what we should. That said, can you honestly say that we have any players in our team who, at this moment, have it in them to perform the way those three Leicester players I mentioned have been doing consistently throughout the season?
Rooney had it in him at one point, but he's been a busted flush for years. Mata is a decent player, but if his time with us has shown us anything, he's a piece that works best in a functioning machine, and not a catalyst. In midfield we've had high hopes for Schneiderlin and Herrera, who have looked incapable of performing at the highest level this season, for whatever reason, while both Carrick and Schweinsteiger's peaks are in the rear view mirror.
In defense we have a Smalling who's been as good as we could have hoped, or even better, this season, while Blind is quite clearly a liability at centre back at this level, while Shaw's injury has been a blow, and Darmian possibly could be a decent player long term, but I doubt he'll be another Evra (who was of that potential match winner level).
In summary, I think it's fair to say that even at the start of the season, we were putting all our chips on Rooney to rediscover the form of years gone by, and Memphis to become an instant hit. At that point, we would have been at the bare minimum in terms of match winners in our squad. As it stands, both of those bets failed, miserably, and Martial has stepped up more than anyone could have expected and saved us from absolute disaster.