The reason I singled out eCommerce is that because once a person starts using a shopping cart, or logs in, Cloudflare can no longer cache the pages, as they have custom content somewhere on the screen specifically coded in the HTML for that user.
Because of this, each requests for the HTML page itself has to go back to the server. That’s usually the biggest slowdown.
Other than the HTML, Cloudflare will cache the CSS/JS/Images close to the users, but that won’t make up for a server that takes 8-10 seconds to respond to HTML requests.