Wait a Few Hours
Often times we see problems strike right at 10AM when a major update goes live. If you want to avoid these issues and long download times, we recommend waiting a few hours for the rush to die down.
When iOS 12 is released, Apple's servers will get slammed with requests. The company's improved the delivery over the years, but we still see server issues and iTunes issues popup from time to time.
If you go to install the update right when it's released, you might see a server error or a lengthy download upwards of 20 minutes.
If you don't have the patience or time to deal with issues like these, you'll want to wait until later in the day to initiate the iOS 12 download.