I run the same test on my test site (gamernewsletter.com) with gtmetrix and webpagetest
on the 5th of July and yesterday
The result for the First Byte time was good (300-400 ms) on the 5th of July and now is poor (more than 800ms). This is the direct consequence of the main page load time 2x slower than at the beginning of the month.
I did not changes anything on the site or either on Cloudflare settings and the host is the same. As far as I understand the file is supposed to be cached by Cloudflare once it is accessed so I could not find any valable reason for this increase.
See the Gtmetrix compare