I have never had a build run good without a hard reset. Both of my Lumia's are done with the upgrade and are hard reseting right now.
Each and every build ran AWFUL on my 1520 and 640 until I did the hard reset.
See, I have to TRY to update first, I am talking an easy 2 hours setting up accounts (3 email accounts, Facebook, Twitter, and 3 other accounts), marking apps for install, configuring each app for what I need, side loading apps I need, etc.
I really want to avoid that hassle if possible, if all else fails, it's try again with a hard reset or roll back to 8.1 again...
It really is a major hassle to do a hard reset and start from scratch, never mind all the game saves I lose (some actually support app backup, SHOCKER)
Still installing....10166> now waiting for 10512 to finish up, gears, looks about 90% or so done....I am sure it will take a bit...leaving in the QI charger through the whole install...