Hey there! There isn't anything that can be done on the cPanel side of things for this, but you may be able to optimize your site code or services on the machine to help with this. We have some additional details in our support article here:
Symptoms When testing a site's TTFB, it is high only when accessing the site via PHP content. When accessing the domain via a simple text file, the TTFB is low. Description In almost all cases of ...
support.cpanel.net
so could you check those out and see if they help?