I don't know how you are satisfied with Juventus' performance. Did Juventus play well in the FIFA Club World Cup? I don't think the team played well. They lost two of their four matches. Juventus won against two weak teams like Wydad and Al Ain. But Juventus lost against city and madrid. Juventus lost against City by a big margin of 5-2.
Yes, I think they played very well against Real Madrid. Especially the Juventus goalkeeper, he deserves the highest score. We have to realize that teams can't always be stable. Another condition is that it's the beginning of the season, I think the coach is experimenting a lot to find the best combination.
Against Man City yes they did perform badly, even if they should have played like when they met Real Madrid the results would have been different. I think this condition will make Juventus think a lot to improve the condition of the players. This is because the transfer season is also still open, so Juventus can still take advantage of it.
I am also surprised to see Juventus on a poor performance against City. Maybe, they didn't take the match seriously because they know that they will qualify to the knockout stage. Juventus played with all seriousness in the round of 16 against Madrid, but they failed to take advantage of their chances in the first half to find the back of the net. Madrid dominated the second half and made proper use of it.