Alkir are you referring to "Auto Minify" ?
I suppose it can improve the load time for JS, CSS and HTML on slow connections as Flarum isn't minifying them. Technically, the additional work to minify that data might make it a bit slower, but I can also trust Cloudflare to be super efficient at that. Some testing with and without the feature would be interesting.
However once Flarum is loaded and you browse between pages, only API (and images) requests are made and those aren't impacted by "Auto Minify", so likely no performance change there.
I have never used that feature on any website myself.