As soon as I read your title, my first thought was Dell.
The problem you are having is a very common problem in many different Dell laptops. For no logical reason, severe power limit throttling can be triggered which forces the CPU down to the minimum multiplier which is 8 for an 8750H. Some CPUs have a minimum multiplier of 4 so those laptops get locked to 400 MHz instead of 800 MHz. The turbo power limit that is being set so low is managed internally by an embedded controller (EC) as far as I know. The power limit settings in the ThrottleStop TPL window are over ridden by the power limit programmed by the EC. Whichever power limit is set lowest is the one used to control your CPU.
Solution? None as far as I know. The last person that complained to Dell was told that this throttling was by design. Technically that is true but something is very wrong when you pay for a CPU capable of running at 4000 MHz and Dell decides that it is OK to lock it to 800 MHz instead.
If this never used to happen when your laptop was brand new then it is likely that a temperature sensor somewhere has failed. Not the CPU temperature sensor. There are other sensors like one that measures the temperature of the back side of your keyboard. When one of these hidden and undocumented sensors detects a high temperature, it limits the available CPU power which forces the CPU to slow down. Nice idea in theory but in practice, the excessive amount of throttling this causes is ridiculous. Dell laptops with 8th Gen Intel CPUs are the first ones to experiment with this new type of forced power limit throttling. Their 7th Gen laptops used multiple throttling methods that ThrottleStop could solve. There is no way to solve the new and improved method.
You can contact Dell but the people working for them on the front lines are not likely to fully understand this problem let alone admit to it and do anything about it. Do what you can to get this problem escalated to someone that fully understands it. A ThrottleStop log file makes this problem crystal clear. You can see the non stop PL power limit throttling messages in the log file and the reduced performance even when the CPU is nowhere near the 100°C Intel thermal throttling specification.