When we say computer / PC performance it is sum of all the activity that is happening in your computer. Right from Network throughput to Printer Activity to Processor performance.Looking at the task manager you cannot find anything but here is a small tool, DiskLED, which can display performance counter data using an animated system tray icon.
Using this tool is straight forward. Once launched, look at your system tray for small blinking / black box like icon. Right click to configure and you will see the screen as in the image above. Here you can select what you want to monitor and what is the counter for it. Like I am taking a look at Network Interface with counter Bytes/Sec and selected the right Interface which in my case the LAN.
After you you will have to rebuild and save the settings. Done that you will start seeing the blinking activity which if you find usual can start looking into the logs for further details.