File Explorer takes 4-5 seconds to open after updating windows to version 20H2.

Hello Null_Pointer,

Try these steps and see if that helps:

1. Search for 'windows powershell' using Cortana or Windows Search.

2. From results, choose Windows PowerShell and select Run as administrator.

3. Then type/paste following and press Enter key:

Repair-WindowsImage -Online -RestoreHealth

4. Let the command complete, restart your system then.

If issue still persists, perform clean boot:

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

Let us know if this helps!