Fix Windows 10 Slow Alt-Tab
Step 1:
Go to Windows Settings (Win+I)
Step 2:
Click System
Step 3:
Click Focus Assist (on the left hand panel)
Step 4:
Automatic Rules (scroll down)
Step 5:
Set “when I'm playing a game” to Off
Video Tutorial: https://www.youtube.com/watch?v=f9VAvT1XZoc