There is no exact answer, it could be any number of things.
Add an .HTML file to the site and see what the response time looks like there. If that is fast, then there could be a bottleneck with your MySQL server request. Then load testing php and MySQL requests to see if it's a send or receive request that is causing problems. Keep in mind that Wordpress themes can play a role too depending how many requests they have.
Then there's DNS, Time to first byte, SSL issues, redirects, general server configuration, etc.