Hi davide
There is nothing I can see that is obviously wrong in the setup or site.
The number of posts and pages at nearly 3000, whilst high, is not excessively so. I looked at your server and tested that, from a data center in Milano (to keep distance lag to a minimum), with S&F active and without (for comparison).
Even without S&F, the site is very slow to load, and well outside Google recommended maximum load times at 6-7 seconds for page load.
Having said that, with S&F active, another 23-24 seconds is added. I tried by disabling Auto Count (which adds to the load time), but that made little difference. Are you able to add the Query Monitor plugin so we can see what is taking the time? I have a horrible feeling that something on the server is the issue, but we will see.