Try using RealTemp, CoreTemp, or HWMonitor to determine this.
If your CPU gets very hot during gameplay, it will slow down to keep itself from burning up.
Considering this is a laptop, you have two factors against you. Laptops aren't designed for the most efficient heat dissipation. Also, the Celeron processor is not great for gaming either. So it's probably working fairly hard, which means more heat. Plus, your processor is also what drives your integrated graphic system. So the CPU is working double duty as it is.
When using your laptop, especially for gaming and such, make sure it's on a flat surface. In your lap, on the bed, on carpet, or other places where fabric is involved you can unintentionally reduce the airflow coming out of the vents on the laptop's case.