1
Open the old Control Panel.
At the top of the old Control Panel, set view to large icons..
Open the Mouse applet.
On the resulting utility, try adjusting the double-click speed setting and apply that to test if that resolves the problem.
2
If that does not resolve the problem, open the Settings App.
Go to Apps - Installed Apps, scroll down to find Notepad, select it, choose Advanced Options.
Run the Repair and the Reset options.
If that does not resolve the problem, uninstall Notepad.
Then download and install the latest version of Notepad from the Store.