Fedora has a long list of mirrors for updates. Some of these mirrors may be unavailable (e.g. Russian Yandex mirrors from Ukrainian IP addresses), some of them may have other issues (like high traffic cost).
How can I block particular mirrors from using them by dnf?
You can disable the metalink= or mirrorlist= line in each .repo file and hard-code a baseurl= line that points to your preferred mirror(s). There’s no way in the dnf settings to exclude a specific mirror. (yum-plugin-fastestmirror did, and the functionality was moved into dnf but the config options were not.)
Also, check the dnf documentation for the “bandwidth” and “throttle” settings.
