Flatpaks are horrendously slow and do not support parallel downloads. I think Discover tries to download updates in parallel, which essentially slows the updates to a trickle at best. According to the hive mind at Reddit, they say to use a VPN (I did, no difference) or to change your DNS provider (tried several, still crap download speeds).
I've also ran into Discover just simply not starting. As in it never launches. No processes hung, no errors, just doesn't start. Reboot, then it will work ok a couple times, then it just stops again. Can duplicate this with multiple machines. Been a long long long time since I used Discover last (way back with Kubuntu), but I do not remember it being this terrible.