Usually text files open almost instantly, but since upgrading to Windows 11, I have noticed a delay when opening them.
The same text files open almost instantly with File Explorer.
Are you talking about opening them with a double-click or previewing them?
Double click.
A Process Monitor log of what happens when the file is double-clicked (plus the name/path of the file, so we know what to focus on) might reveal what's going on.
Include both file and registry data in the log as it could be important for this.