WP core and admin-ajax slow down the website

Hi,
After some little researches (as I’m a newbie on wordpress) with P3, I’ve found out that my website has got some core issues that slow down the website.
First I suppose that the problem was on the content of the pages but I realized that even running the admin backend, it suns very slow.
P3 showed me that the core loading is one of the most guilty and admin-ajax.php in particular.

The website is

How can I fix this?

Thank you so much!