Pagespeed is relative so if your origin web server is located further away from Cloudflare Edge server/visitor location, then page speed and round trip times will be slower. If you web host origin server is closer to your visitors and test location, then it will be faster.
This is due to Cloudflare not caching HTML content by default (see below). This means your dynamically generated HTML content will be served from origin as it is not cached. So how fast a page is will be determined by your origin web server/php processor speed. So tuning and optimising your origin server will help.
So for optimal speed, you want your origin real web server to be hosted in a location closest to your majority traffic visitors and then put Cloudflare in front. For instance, my forums has 50% US visitors 40% Asian visitors and 10% Oceania. So my optimal geographic location for my origin is US West Coast as it sits in middle of US, Europe and Asian so equal round trip times for majority of visitors.
Cloudflare cache certain static content https://support.cloudflare.com/hc/en-us/articles/200172516-Which-file-extensions-does-Cloudflare-cache-for-static-content- but not dynamic/static generated html itself by default. But you can tell Cloudflare to cache dynamic/static generated html content to some extent depending on Cloudflare plan you’re on via cache everything page rule or use Cloudflare Workers with custom cache everything for guest visitors only. But have to be careful to only do this for static html content and not dynamic html content (otherwise you would cache private logged in user content).
I’d test your web site with/without CF via webpagetest.org as they have alot of geographic test locations and allow you to test real mobile devices and different browser clients and connection speeds and test and reports those additional Google focused pagespeed metrics.
Share your WPT result urls for both with/without CF with us.
I wrote a guide for my users which maybe useful to you as well on my forums at https://community.centminmod.com/threads/how-to-use-webpagetest-org-for-page-load-speed-testing.13859/ (yes using Xenforo myself )