I'll assume that you're using Windows 10. Here are the things that I'd try:
Step 1: [NB: You can try skipping this step by going straight to Step 2. If Step 2 won't work directly though, come back and do this bit first]:
Boot into Safe Mode by following the instructions here:
https://support.microsoft.com/en-gb/help/12376/windows-10-start-your-pc-in-safe-mode
(Note that your screen might look rather odd while in Safe Mode. That's due to the display driver not being loaded and is perfectly normal).
Step 2: Perform a disc scan by following the instructions under 'Method 3' here:
https://www.laptopmag.com/articles/repair-windows-10
(NB: Note that there's a
space between 'sfc' and '/scannow'
Step 3: Restart your computer to see if things have improved. (That will automatically get you out of Safe Mode).
Step 4: If things haven't got any better, try using System Restore. (Your post suggests that you know how to get there, and that you've already tried this option. If so, give it another go while in Safe Mode)
Step 5: If there's still no joy, launch Windows Task Manager, ensuring that 'Show processes from all users' is selected. Look for any process with high CPU usage and try killing it. Repeat for any process that's taking up loads of memory. (Tell us the name of any dodgy-looking process if you need help in working out what it is that's actually causing the problem).
Step 6: If you're still getting nowhere, try disabling your security software. (Google can probably provide you with instructions. Otherwise please tell us which security software you use). It's not unknown for security software to get 'stuck in a loop' when trying to update itself, causing the host computer to run at a crawl. (Windows Defender is a prime culprit but other security programs seem to have the same problem occasionally).
Step 7: If you're still tearing your hair out, go into the Windows update history and try uninstalling anything that's been installed recently.