An F-Droid Repository is just a collection of static files served from any HTTPS webserver. So all the questions of loadbalancing, speed, hosting, etc. are separate from fdroidserver. fdroidserver is perhaps not the best name since it really is a collection of tools for assembling and publishing collections of static files.
We’d love help getting our hosting setup working better. It is already load balanced across three instances behind the f-droid.org domain, as well as numerous mirrors. To dive into this topic, see the thread https://forum.f-droid.org/t/slow-webserver-response/