My game was running fine, then i upgraded to windows 10 and it has become unplayable. 4000+ ms and game freezes whenever i get into combat. When it's running well, i have about 300 ms and a 1-2 second delay on every action. I'm currently trying to reinstall the game to see if that helps, but I don't know what else to do.

This more or less sounds like An ISP issue try doing a tracer route see where packets are being lost

If it is not your company then it may be A bad gateway on blizzard's end

If that does not solve it then Reinstall windows 10?

Make sure everything is Updated

Windows 10 made my lagg horrid due to a driver malfunction.
Try uninstalling Ethernet / wireless drivers, and reinstalling them.

Delete system 32