The gaming performance of any PC is determined by its processor (CPU), graphics chip (GPU), and Random Access Memory (RAM). If you want to know which one of your CPU or GPU limits your performance, follow the lead.
How to test your PC hardware?
We are going to test the number of FPS (frames per seconds) you can achieve in a graphics-intensive game at different display resolutions screen resolution.
Top-notch configuration can run triple-A games over 120 FPS. The closer you are to this number, the better it is. You can use a program like FRAPS to measure the frame rate of your games.
Adjust the display resolution (maximum) and check how the game behaves. Please note that display resolution is expressed in pixels (width x height) and it has nothing to do with the level of details that can be achieved in the game. You can adjust the maximum display resolution by going to the Graphics options/settings via an in-game menu.
Play for a few minutes and then record the number of FPS achieved. Repeat the operation at different display resolutions.
How to analyze the results?
If you notice a significant improvement in terms of frame rate when running the game at a lower display resolution, then it is most likely that the performance of your computer is being limited by your graphics card (GPU limited).
If you didn't notice any significant improvement in the FPS even after lowering the display resolution to minimum, then your processor is faulty (CPU limited). No doubt it is too old or too cheap to run the game.
If the game is slightly better at a lower display resolution, then it could also mean that both of the components are faulty. You're both GPU and CPU limited.