We also observed this after the upgrade to v7.5.0 and even though we have relatively resolved it as to deal with the huge CPU spike when the domain registration page is visited, it is still not fast as it used to be.
There are a couple of things you could try server-side.
First, if you are using cPanel, first disable or remove or reinstall mod_http2 and see if the issue persists.
Secondly, if you have the new mod-cpanel compiled, disable or remove it to see if that will stabilize the server.
Check your error_log(s) after each of these steps.