I can afford to take the time to investigate potential problems these machines have because I've sold 3 of them to my clients and assuming responsibility to supply a properly working device. Sadly this is quite unlike how this industry is generally working.
As to your framerates, there is a lot involved in that and I can't answer your question without investigation, and especially since I don't know any details, such as even the game type, what sort of scenes the game involves and whether there is any correlation between the performance drop and the scenes, how your power management preferences are set up etc. Generally, this might be CPU or GPU throttling but then not every thottling means that the device is not working properly. The CPU and the GPU in almost every modern laptop (except for ones with really weak chips) is capable of providing more performance than the cooling system can cope with. This is just a reality of practical laptop engineering. So basically performance in laptops is limited by the cooling system. Moreover, it is limited adaptively in real time, with many layers of firmware and software governing the ultimate performance by slowing down the chip when necessary. Slowing the chip down for any reason, including thermal, is called "throttling". So from this you can see that throttling is a fact of life in all laptops and that is normal. Moreover, "normal" throttling can occur because of other reasons, for example because of preferences set by the user, for example if the user wants to sacrifice performance for less fan noise or longer battery life.
Now there are cases where throttling is not normal, for instance when a buggy software reads out some sensor and decides to slow down the system for not necessarily a good reason, and then doesn't recover from that state. Or when the software that is operating the cooling system is not smart enough to adjust to varying loads in real time. Or when some driver is not optimized for a specific game. There is much more to this.
To sum it up, probably you are experiencing throttling but I can't tell what would be the reason to that - is it thermal? Is it malfunctioning software? Is it your preferences? Is it an issue with your charger? Or is it just the game being too heavy for the hardware or not optimized properly?