What is input lag? 

Input lag is how long it takes (in milliseconds) for a monitor or TV to react after you input a command from your keyboard, mouse, etc. 

Troubleshooting for Input Lag

1. Switch to a wired keyboard and mouse, or controller. This will ensure that wireless latency or interference isn't a factor in your situation. If switching to wired doesn't improve your input latency, you can switch back to your preferred peripherals.
2. Try lowering your in-game video settings to reduce the amount of work your computer's doing. This can reduce input lag in some cases.