Do you have latest drivers/bios for PC? that could be one reason it gets stuck. How new is the computer?
One option is to download the
Windows 10 media creation tool and use it to make a win 10 installer on USB, and then try to run the update off the USB. No more downloading 4gb multiple times to have it fail.
If update still fails, I would suggest a fresh install as often its only way to bypass windows update. It might also fix the slowness.
fresh install steps:
Backup everything you want to keep as this will wipe hdd C is on
change boot order in BIOS so USB is first, hdd second
boot from installer
follow this guide:
http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html
when you reach the screen asking for licence, click "I don't have a key" and win 10 will continue to install and reactivate once finished
On the screen where you choose where to install win 10, if it gives you an error about GPT drives, delete all the partitions on the hdd and press next. If it still gives error, cancel out of the installer and restart PC and start installer again, it will accept next on that screen this time (some PC just need a restart here).