That is a good point that you mentioned here, but I was rather referring to the World Cup disrupting the momentum that Napoli has in the current situation. In the past one could observe at times the these breaks are also kind of a break to a certain momentum of a team. Of course that doesn't necessarily apply in any case, but it has happened in the past.
Now Napoli is totally on top of everything. At least in the Series A they seem to be unstoppable for the moment and they probably will successfully keep that level at least until the World Cup starts. Afterwards we'll have to see whether they can seamlessly continue their streak.
Of course Napoli will benefit from the lack of participation of their players in the World Cup, but that doesn't guarantee anything about whether they can maintain their form. The long holidays during the world cup can bring some players down due to their activities and lifestyle, this has happened to some of the top players before so when they come back the performance is never the same.
So far Napoli are unbeaten, convincing as one of the title challengers, but the fact is they have not been able to win the Scudetto in many different seasons despite their good performances. It's only about 12 match, there are 26 more match that will await them and I think they will also have other focuses like the Champions League or the Italian Cup.
I guess Napoli will be able to maintain their consistency after the World Cup. You say long vacations will degrade the performance of the players due to lifestyle and activities. But I don't think so. As they are professional athletes. So they will definitely be conscious about their fitness. And I think their performance will not deteriorate if their fitness is right.
As Napoli has been able to continue their good performances in both the Champions League and Serie A so far, they will not want their performance to suffer after the break. They will definitely be aware of this.
Juventus had a lot of problems winning against Lecce. Their performance is still as weak as before. They are not able to perform well even against relatively weaker teams. If the attacking performance is not stronger, Juventus will not be able to climb the points table.