Even with good internet speed, sometimes the w10 upgrade process get's stuck.
I had old w7 laptop that was not up to date (stayed off for several months), so it took a lot of time to install latest updates.
Couple of months ago I reserved the windows 10 upgrade but i didn't allow enough time to laptop to install all pending updates, i just shut it off.
Once all updates installed (some, older, were shown as 'Failed', not sure why), it asked about upgrading to Windows 10 and it started the download process for windows 10.
This process got stuck several times, tried restart, etc, nothing worked.
And was not moving past a certain percentage 6 or 20 or 30 - not sure...
I left it for hours and it didn't move further, it was just stuck...
So I fixed mine by deleting the contents of this dir:
C:\$Windows.~BT
C:\Windows\SoftwareDistribution\Download
restarted, then it asked again to upgrade to windows 10, i confirmed then went to download the windows 10 update.
It took quite a lot (2 hours?) and at certain point i thought that again it's stuck. But i left it and then it finished and started the process (which btw, was very quick to upgrade to windows 10)
hope this works for you.
-s