UsersWP lightbox form loading slowly

I am using UsersWP with the login lightbox activated.
After I installed The Events Calendar it takes ages for the content of the login form to be displayed. With the lightbox deactivated the login page displays immediately.

Based on what I found searching the internet I also tried changing the setting of AyeCode UI-> Fontend Load JS to something else than the default Core + Popper.
The result is that the lightbox does not display the login form whatever the setting of the Login Lightbox is in UsersWP.

No Javascript errors appear in the console, but admin-ajax.php seams to be the culprit with a loading time of 12.63s !