Hi and thank you for your response.
Deactivating the revolution slider have no impact at all to the problematic loading behaviour.
If we replace all the XStore plugins replacing it with others, the speed is returning to normal.
We haven’t tried WP Rocket, we are letting it as a last resort but hopefully another solution will come up and we won’t need it.
Strange thing is that in Google Page Speed we are getting a decent score when Inspector -> Network shows 28,5 seconds to load the first page…
Anyway, if you have any other suggestion please let me know.
Thanks again.