Friend & I had same problem - driving us nuts - It apparently has something to do w/ problematic DNS servers in Western US - found help here: Yosemite download is insanely slow (nothing wrong with my network)
- See Frank252 & ryanbr responses @ above URL.
Here's what I did & it worked for iPad2, iPad Air2, & iPod 4, using iTunes on a Mac-mini to load iOS 8.3:
1. Reset the DNS servers (per Frank252) - using System Preferences/Network/(Ethernet or Wifi)/Advanced /DNS - to the Google servers: 8.8.8.8, & 8.8.4.4. Note your original server ID's, so you can revert if you want.
2. Clear DNS cache per OS using terminal commands as described by ryanbr:
- Mac OSX 10.10 (Yosemite) sudo discoveryutil mdnsflushcache
- Mac OSX 10.9 (Mavericks): dscacheutil -flushcache;sudo killall -HUP mDNSResponder
- Mac OSX 10.7 & 10.8 (Lions) : sudo killall -HUP mDNSResponder
- Mac OSX 10.5 & 10.6 (Leopards) : dscacheutil -flushcache
- Mac OSX 10.4 (Tiger) : lookupd -flushcache
You may get away w/ skipping this step - try first without, if unfamiliar w/ terminal command process.
3. Plug iPad into Mac-mini, bring up iTunes & initiate update - I started out w/ download only, beings I didn't trust the SOB to get the job done in a reasonable time after screwing around for a couple of days w/ this. Anyhow, download time went from >>20hr (stated time) to ~ 30 min on 10Mbs DSL.
4. Initiate install on iPad per iTunes.
I also had slow download for OS X/Yosemite updates, & this did the trick for that also.
If you want to do the update directly, wirelessly (ie, w/o computer/iTunes) on your i-device, you would likely need to do the DNS server change at your router.
- Hope this is clear enough to help - good luck.