Technical Support - Keyboard Input Lag

A friend of mine recently purchased a PC after being a laptop gamer for many years and has been having issues with his keyboard input lagging (mostly noticeable during games).He had a Razer keyboard, but as of a few hours ago, he's bought a cheaper one just to test if he was having the same issues.

He's tried:
- Uninstalling and reinstalling the keyboard drivers
- Turning on and off V-Sync within the games
- Uninstalling Razer Synapse and Cortex (something I suggested because I had issues with them)
- Using different USB ports
- Purchasing a new keyboard
- Updating his graphics card

Any help that you can provide is greatly appreciated.