What? What is that, what causes it, and how to fix? Throttling? What throttles what?
Your CPU gets hot, we already knew that. Modern CPUs are designed in a way that they have a dynamic frequency, which is called “boosting”. If they are within thermal and power and voltage limits, they can increase frequency. I am not sure that your CPU even has a boost, but if it gets too hot (95°C) it will throttle/clock down (reduce frequency). Same goes for GPUs (the chips on graphics cards; both the boosting as well as the thermal throttling.
It would be interesting to monitor, e.g. with the Riva Tuner Statistics Server coming with the MSI Afterburner, which of the two (if not both) is the culprit. Which of them is responsible for your fps going down (and going up again after a short breather -> the menu is easier to render/maintain).
Then you could try and find the settings which are most intense on this component.
Now that really goes in-depth and I am not sure it will be successful after all. I am just trying to explain how one would go about to see if frames can be lifted a bit.
How can I tell if it has a boost or not? 
It actually does:
Look for “Max Turbo Frequency”. The max boost is always only held for a certain time, but usually the sustained all core frequency can be thermally dependent. And on top of that it will always throttle if you reach the max allowed temperature.
Good thing is you already have this cooling pad from your nephew. You can test whether it makes a difference without spending any money.