My Win10 update 20H2 is 5 times slower than before...

Hello HuggySW, 

Suggest you to run System File Checker scan:

1. Search for 'command prompt' using Cortana or Windows Search.

2. From results, right click on Command Prompt and select Run as administrator.

3. Type sfc /scannow command and press Enter key.

4. Let the command complete and then reboot the machine.

If issue still persists, perform clean boot:

https://support.microsoft.com/en-in/help/929135/how-to-perform-a-clean-boot-in-windows

See if it makes any differences.