I’ve noticed that login takes around 10 seconds on my Mautic 3.0.1. I guess this is a protection against brute forcing the password.
However, 10 seconds may be too long for the the HTTP server to wait for a response from php-fpm. If this assumption is correct, you have to look into your HTTP server configuration (Nginx?) to fix this. My guess is that fastcgi_read_timeout is too low.
I’m not familiar with cPanel. The first thing to do is to determine which HTTP server you’re using: Apache ? Nginx ? and then look for the gateway timeout parameter specific to the server you’re using.