Had the same problem, posted a topic in another vcmp-related forum, dudes over there told me its a virus and its a completely different thing, so from now on if you have similar problem never dare to post here. I already saw answers and they are totally irrelevant and gibberish.
so let me explain to you the phenomenon:
you have stable 60 fps at beginning, that's normal and then your computer starts to slow down until it gets unplayable, you are very confused and don't know what is happening, actually your computer is getting overheated, what does overheating/power throttling/ gpu throttling means? google it because it'll give you a very detailed explanation.
How to fix this problem?
1- type in google "how to fix gpu throttling/power throttling" and it will give you a set of steps you should do inside windows to adjust power plan and prevent overheating issues
2- Clean your central unit from dust and any particles that might block vents entrance (I recommend this solution)
3-If nothing works, buy new fans.