I don't say anything about the last matches Barcelona when they know they are going to win the La Liga title because during that time they didn't have enough motivation to have a better performance, for example, in the last game Barcelona had against Celta de Vigo they lost the game with a 2-0 result but it doesn't mean Barcelona couldn't win that game, the thing is they didn't need the full three points from this game.
I guess in the next season Xavi will even have a better perforate in Barcelona with the more experience he got.
Barcelona's performance has indeed decreased since securing the League title, but for Celta Vigo this victory is very valuable to ensure their place in La Liga next season. At first, Celta Vigo, if they lose, there is a possibility of relegation, so they are very happy when they get three points.
In the end, when you look at the results of other teams' matches, winning and losing Celta doesn't become relegated. but still if you can't beat Barcelona, Celta Vigo is ranked 17/20 and when they win they get three pounds so it's 13/20. What's interesting is that Real Sociedad, Valladolid and Celta have Barcelona to thank. because Barcelona have helped all of them in their goals, especially Celta Vigo and Valladolid in their fight for survival in laliga. Although in the end it was Celta vigo who stayed in the league, Barcelona also helped Real Sociedad secure a place in the Champions League.