Have your checked Thermals? That seems like the most likely cause, since it can throttle your frequencies and consequently worsen performance overall.
If you're not sure how to do this simply download a program called MSI Afterburner and look at the performance metrics after using your Computer the way you normally would.
You're looking for temperatures that seem exceedingly high or frequency drops during intensive tasks.
If it is in fact the case that your system is Thermal Throttling, you might want clean your Computer from dust that accumulates over time and replace the Thermal Compound on your Processor.