If the problem started recently and suddenly, something was installed and is running itself in the background, eating up CPU cycles and memory. Check the task manager first. The "lag" should only be noticed for tasks that previously ran smoothly if that's the case.
If so, roll back the registry with system recovery during start up to uninstall whatever is currently hogging resources.
Worst case scenario if some unwanted malware code/app is running in the background and system recovery can't remove it, back up, reformat HDD, reinstall OS, reinstall apps. Hope that's not the case.
Commercial system tools like Defender Pro or System Tools often identify common problems and can optimize the operation of your system.
If HDD is defragged, check that box.
If you are running current VGA drivers, check that box.
If it's a new game(s) he's trying to run and his computer is "lagging" as it's already been said, the specs of his system may simply not be up to the task of running at the settings he wants. Time to upgrade hardware or run at reduced settings.







