My understanding is that F-Droid is hosted out of some home servers (instead of some universities like other similar package managers) so the bandwidth leaves much to be desired. But the UX is definitely a big part of the problem. I don't understand why it tends to abort downloads when I background it, and I don't understand why it doesn't show a toast that it aborted the download.

I very much prefer Obtainium these days despite the setup steps. I don't think it's a coincidence that Obtainium, Aurora, Zapstore, etc. are gaining mindshare over F-Droid, just like how Brave has explosive growth over FF.