The static site hosted on my Pages with Pro plan seems always slower than directly hosted them on Github. It’s slower from time to first byte and total time when I tested them using webpagetest.org and I tried multiple locations.
Does your static HTML files contains links to 3rd party resources? In general all files are static, Cloudflare would keep a cache of the HTML, but if you have resources that links externally to a different hostname for files like jquery, js, css, boxicons etc, they could delay the request.
I think everything is local and I host identical files on Cloudflare Pages and Github Pages.