This type of thread happens within minutes every time there is an iOS update released.
The problem is simply that Apple has "X capacity" on their servers. Once the update is released, every Apple owner on the planet tries to download at once, exceeding the available "X capacity" and (technical term coming) clogging the works. 😀
For a smooth download experience, just wait a day or two. By then the capacity will be available. Probably not reasonable to expect Apple to build capacity out to cover the capability for downloading to everybody all at once.