Why is my girlfriend’s gaming laptop performing poorly all of a sudden?

It sounds like there might be multiple factors at play here. Have you tried updating the drivers for her CPU and GPU? Sometimes outdated drivers can cause unexpected performance drops, especially with newer games. Also, make sure to check if her laptop is detecting and utilizing the dedicated GPU, not just the integrated graphics. You can check that in the Task Manager under the Performance tab.

Sometimes, the problem lies with the games themselves. Have the games she's been playing recently received updates? Certain updates may increase the system requirements or cause compatibility issues. You might want to run her games after verifying all drivers are up to date and check if any are running unusually high in resource usage.

I had a similar issue where the laptop only recognized the integrated GPU instead of the dedicated one. Make sure you check the graphics settings in the manufacturer's control panel and the game settings. Also, if you've run a malware scan, you should give Malwarebytes a try, as it can catch things that Windows Defender sometimes misses. Also, consider disabling unnecessary startup and background applications to free up memory.

Could it be an overheating issue? You mentioned temperatures around 40°C, which is fine, but is that under load or idle? If the laptop's thermal management isn't functioning properly, it could throttle performance even if the temps seem okay. Also, double-check that the power adapter is working correctly; a failing adapter could lead to power inconsistencies.