I don't think Barcelona's performance is getting worse at this point, when it comes to football we believe that Barcelona cannot compete with Real Madrid because they don't have good squad as Real Madrid did. Barcelona's performance was very good in El Clasico but they cannot maintain the winning way to the final whistle. I think if Real Madrid had a striker in their game against Barcelona they would score more than three goals in the game because Vinicius was missing the chance to score more than three goals. I still consider Barcelona a strong team because they perform well in each other their games.
If Xavi changes his decision to remain in the club next season, the most important thing they need to beat Real Madrid is to sign more players because if they rely on the same players next season, they won't achieve anything, since their current players are not strong to challenge Madrid for the title next season.