Why IOS update download is so so slow ?

I have home internet download speed of 3.6mb/sec(with Spectrum) and AT&T wireless speed of 45Mb/sec. Tested with Speedtest app. I started downloading ios 11.3. I know it is a big download but taking over 17 hours and still going. I tried using both(home,wireless) on iPhone 5S. IPhone was Reset than updating from ios 10.2 to 11.3. Why taking so long ? Is it because when I reset the iPhone, I said skip adding appleid ? Is is possible that I started with slow home internet speed and Apple download server thinks my speed is very slow ? Does Apple stream download from different servers(fast,slow) based on what ? How to speed up download ? Is there a way to pause download in-middle and move to faster internet speed or download servers and don't have to start all over ? When you expect the download to finish in few hours and when takes over 17 hours and still not done than it is very frustrating.