Their update servers have been slow for years. This is normal.
Using Verizon FioS?
I had a terrible issue with Fios and trying to restore my iphone back to iOS 8, it kept saying "500 hours to download" or it would just not work
Went to the Apple store to get it restored on their connection and it was done in 10 minutes, if your on Verizon Fios, its them, they are doing some ****, throttling, doing something, many Fios users are complaining
Next time change your DNS servers. Verizon still has issues with out of date DNS info since Apple changed their servers.
I did all that, I changed my DNS to Google's DNS, 126.96.36.199 and it still wasn't working, funny thing is, at the Apple Store the Genius said the same thing , that on his Fios connection it wasn't working but when he went to his girlfriend's house with Comcast it worked instantly and fast.