I guess it is has ancient download algorithm from the days of dial up modem. Even the download addons on Firefox have had this capability for several years now. All download utilities and apps have had this capability for more than a decade. Unfortunately the Apple engineers don't have a clue about it OR they are not smart enough to include that.
For some reason my wife's iPad 2 stopped charging and alternately displaying red battery outline and apple logo. Been trying to download the1.33 GB OS through iTunes. Currently I am on 4th attempt. It simply stops around 1.2 GB. I paused the download. Switched ISP, rebooted my modems + routers but it every time started from zero. And this is on 60 and 100 mbps connections. These work without any trouble for torrents and http downloads just not with iTunes. Won't even allow me to use third party download software.
My wife needs it only for Facetime with my granddaughters 8000 miles away. Perhaps it is time to switch to Google app for the purpose!