Have you monitored your CPU and GPU temps using HWinfo? It sounds like a classic case of thermal throttling to me. It's not uncommon on laptops that have been used extensively for gaming. Laptops simply cannot handle long term high demand gaming, the cooling systems, even the best of them, just don't hold up in the long term. From least to best models, there is ultimately very little difference in the cooling design or capabilities, so when you stuff that much hardware into a confined space with no way to improve the cooling, something has to give eventually. This is exactly why laptops are a poor choice for gaming devices if you plan to use them in the same way you would a console or desktop gaming system.
There are literally thousands of similar threads here and elsewhere with similar thermal degradation issues after prolonged gaming use.