On one hand, we have a HTML website on the server. It won’t change, it’s always static. There’s no background processing going on. You request it, it appears.
On the other hand, we have a content management system driven website and a request for a page uses PHP and a database to “assemble” your website on the fly. PHP assembles the HTML and the database pulls in the content & the extra processing takes time.
PHP and a database introduces additional load time and delays which makes a HTML only website fast in comparison.
A HTML website an be completely customised so your site dosen’t look like some overused CMS or free website template.