I don't think ISP has anything to do with it (at least in my case)
I downloaded the game via Steam in less than an hour - reliable connection, the download speed didn't go down at all.
While using the Game Center it has spikes, but most of the time it's around 100-300 KB/s.
I also tried everything, checked unchecked no uploading, restarted the app, restarted the pc (multiple times), tried both wi-fi and cabled connection. tried to pause the download and resume.
When I'm downloadin anything through a torrent client it's again reliable, no ups and downs.