I recently deleted everything from my PC and reinstalled Windows 8.1 from scratch. When I first booted into Windows everything was extremely slow though the CPU usage was very low (about 1%). After installing some drivers the problem seemed to be solved, I was able to use my PC normally.

Today I installed a game and I noticed a strange behavior: the game was playable but the performance worsened more and more in the time.

This is the situation BEFORE opening the game (normal):

This is AFTER some minutes inside the game (low CPU usage and clock):

Some information about my system:

• PC: Sony Vaio S13 (SVS13A1C5E)
• OS: Windows 8.1
• CPU: Intel Core i7-3520M 2.90GHz
• GPU(1): Intel HD Graphics 4000
• GPU(2): NVIDIA GeForce GT 640M LE

I tried searching for new drivers and other solutions but noting worked and I don't know what is the cause. I did not checked the temperatures but the fans are not running fast and the PC does not look overheated.

Update:

Max CPU Temp: 66°C, Max GPU Temp: 61°C

The strange thing is that the GPU load is 99% (GPU-Z) and the fan is almost silent.

Update 2:

I had troubles with Sony Vaio software, I can't get the FN keys and the STAMINA/SPEED switch to work (it is a physical switch to enable/disable the Nvidia card and change the Power Profile). I'm saying this because I remember that before reinstalling Windows there was an option in the Vaio Control Center (now it is not there anymore) that allowed me to choose from something like "priority to performance (ventilation)" or "priority to silence". The current behavior looks like a "priority to silence", but I can't get the stamina-speed switch to work and so I don't see similar oprions in the Vaio Control Center. I don't know if the problem is related to this.