They don't use one AFAIK. They have mirrors, but the way stuff works is that their main server is still the bottleneck. This has been raised before (https://forum.f-droid.org/t/why-is-fdroid-down-all-the-time/... and a few other threads on the forum) without a satisfactory resolution. As an outsider, to me it appears that f-droid's user base is mostly European, and it works well enough for people in Europe.