This seems to me to be a major bug in Windows 10. But you can stop Windows Update to recover your internet connection. If you go to Network & Internet in Settings, then WiFi, then click Advanced Options, you can set the network as a metered connection to stop Windows Update. I suggest you do this temporarily so that you can actually use the internet.
Here is a link to the issue in the Windows Feedback app, if you want to upvote it:
(Paste it into Edge)
I've started noticing my internet connection being ruined even when Windows Update is not updating. In the task manager, it shows Service Host: Local System using up all my bandwidth. I tried going to Start > All apps > Windows Administrative Tools > Services, finding Background Intelligent Transfer System, right clicking it and clicking Stop. This stopped svchost using up all my internet. (I couldn't try the metered connection trick on this computer as it's connected via ethernet to a range extender)
Not only is it my computer on which the internet becomes unusable, but every other device on the network. I don't know how Microsoft have achieved such a horrific bug.
This might not just be a problem with Windows Update and the Background Intelligent Transfer System. It looks like other background processes are also hogging bandwidth, e.g. I'm currently updating Adobe Photoshop and the Adobe Installer process is using pretty much everything, and web browsing has slowed right down (although not as badly as when the former two are downloading stuff).