I also noticed that PSG’s performance has declined since their defeat to Chelsea in the CWC final. Their recent matches have been less than convincing, and even beating a team like Nantes proved somewhat difficult for them... despite dominating possession and having far better players., they still failed to collect many points from those games.
PSG performance may decline just as you said, but I don't think the decline of there performance is because of their defeat by Chelsea in the club world cup final. PSG is a big club and almost everybody was expecting them to win the club world cup final, but it happens that the club world cup final didn't end in their favour, because they where beaten hands down by Chelsea in that club world cup final. No doubt that PSG recent performance wasn't impressive at all, but let's not conclude that PSG is weak due to their resent performance.
PSG may just be temporary down for the main time as they have not rested enough I see PSG coming back stronger and better because they will definitely learn from the few setbacks that they have had these two days