The first step towards diagnosing performance drops is to monitor your system while gaming. You can use your tool of choice, but I personally like MSI Afterburner with the Rivatuner in-game overlay enabled. You can configure this to allow you to monitor lots of different system parameters while you're playing.
You should monitor the following things: CPU usage, CPU frequency, CPU temperature, GPU usage, GPU frequency, GPU temperature, RAM usage, and storage usage.
If you notice components spending a lot of time at 100% usage, operating at lower than expected frequencies, or operating at high temperatures, then you can further investigate that component.
While monitoring your performance, verify that: your PC is free of dust and that all fans are spinning properly, your display is plugged into your graphics card and not your motherboard, and any programs that might be running in the background are closed.
Let us know what you find and we might be able to offer additional suggestions. What are your specs?