I am playing with HTTP/3 since a couple of days, and I see a speed increase of roughly 10% compared to HTTP/2. However, considering how HTTP/3 is more resource intensive, different results will be observed, depending of the CPUs of the servers serving the files.
For the records (adoption), HTTP/0.9 : 1991 HTTP/1.0 : 1996 HTTP/1.1 : 1999 HTTP/2 : 2015 HTTP/3 : 2019
If this is within the context of dynamic pages, chances are the gain of performance will be insignificant with Cloudflare. Keep in mind that Cloudflare works as an intermediate actor. It forwards requests to your server, which itself sends the response to Cloudflare ,which forwards it to the end user. This means that, most of the load time, still depends on how fast your server is, at generating the page, and how fast it can send it to Cloudflare.
HTTP/3 has more benefits, when the dialog is direct between your server and the end user.