While I sympathise, you can’t expect Microsoft to come up with a special version for those with Internet connection issues, be it lower bandwidth or otherwise.
The last update took maybe 20-30 minutes to download, and install, and at its peak was using half my bandwidth.
I don’t think you stated what broadband speeds you can get, but you did say you are hitting a limit of 15Mbit/s.
My downloads tend to hit around 220Mbit/s, and my broadband is 500 Mbit/s. Steam by comparison regularly shows 65MB/s, because my provider has always allowed a little more to allow for the rates I pay for a peak times, so in reality it’s closer to 600 Mbit/s.