I can't say if Chelsea has improved in their performance or not, it's when the new season starts is when we would know. However, their performance in CWC is impressive and their fate will be tested in the finals when they play against PSG who have destroyed and humiliated top big clubs in various leagues in recent times.
The Premier League is a different ball game from the Club World Cup tournament, and it is more difficult than the Club World Cup tournament, so we can't use the fact that Chelsea managed to reach the Club World Cup final to justify or believe that Chelsea has improved in their performance, so I agree with you that it's when the new Premier League season starts that we'll know if Chelsea has truly improved in their performance.
It's not yet time to judge when the Premier League hasn't even started yet, and will Chelsea be as good as before, or even better? No one knows, but what clear is that with the recruitment they've made in this summer transfer window, I believe they haven't made as many mistakes as in previous transfer windows and are certainly only buying the best quality players, such as Joao Pedro, who was one of the best recruits because he immediately performed brilliantly in his debut with Chelsea in the Club World Cup yesterday. Perhaps with this, they will improve their performance in the Premier League later.