For anyone else having issues with the battle system after upgrading to Windows 10 (or just in general), here's a description of my experience with this:

While in the battle screen, if using the keyboard numbers (1-5) to select my attacks, I was experiencing a 3-5 second delay before I could select the enemy to focus the attack on. I didn't have this delay issue if clicking on the attack icons, though that is a lot slower.

As Krin suggested:

Link: https://www.sylestia.com/forums/?thread=28125&page=1#2 Author: Krinadon Time Posted: 8/24/2015 at 12:17 AM I believe the issue you are having is related to a feature most laptops have that lock the screen when typing so you don't accidentally click/move the cursor around. If you go into keyboard or mouse settings, you should be able to find where to disable it.

I found this under Settings>Devices>Mouse & touchpad. You want to disable the delay under touchpad settings, or at least change it from medium to short.