That SpaceDock is very slow if downloading in CKAN?
Can't confirm that. In my experience GitHub and SD are pretty much about the same speed, but that's probably limited by my 50 Mbit/s connection.
Archive.org is (understandably) definitely the slowest for me, far from hitting 50 Mbit during downloads.

I'll try if I can get some numbers, but I think there won't be much difference between GitHub and SD.