Word Press Page Loading Speed Problem

We have a client website at (PM me for the link) which has an extremely slow page loading speed and we need someone to troubleshoot and fix this problem. The website is on a GoDaddy virtual dedicated server using Word Press (with multiple plugins) on Apache Server running Linux with MySQL database and PHP. This must be a fixed fee for a one-time fix of this website. There are many client websites on the same webserver, so you cannot change the WP version of the PHP version and you must have a test environment where you can copy the site and test the configuration to validate what the problem is BEFORE going live with the fix.