I discovered a nifty trick for those excruciatingly slow Apple Update downloads. My friend left his broken 2008 Mac Pro with me last week. It turned out that he just neglected to plug in the display power supply. The computer has been down for a year(!) because of that.
Since the machine hadn't run since last summer, he was still using Yosemite so I decided to do his free upgrade to El Capitan because obviously that was going to be over his head too.
I started the update and System Update reported that the download would take 3 days, 2 hours. WTF?? I did the same update on my Mac many months ago and remember it taking like 15 minutes. I have a 100Mb connection here.
I kept pausing/restarting the download and the download time didn't budge. Obviously Apple's content delivery network was bricked.
Then it occurred to me that CDNs are usually regional. A user in Australia will be sent to a different CDN node than a user in Italy. Maybe it was just the US node that was farkled.
So I surfed for public DNS servers and found this list:http://www.bestdns.org/.
I went to System Preferences -> Network -> Advanced and deleted all the DNS servers. so there would be no chance for a timeout failover to the next DNS server on the list. Then I added one for the UK that I got from that list above: 126.96.36.199. I wanted an English speaking country just in case Apple defaults the update to the local language too. My Cantonese ain't so good.
My three day download took 11 minutes.
Remember to revert your DNS servers back to what you had before you run the update. Or just use Google's: 188.8.131.52 and 184.108.40.206.