Re: URGENT! My game, The Sims 4, lags severely when I plug my laptop charger in.
You are right, neither your GPU nor your CPU are overheating, but there is still thermal throttling.
The Sensor mentioned (PROCHOT EXT) means that something different then CPU or GPU is overheating or is at least signalling a overheating problem.
That part might be the super input/output circuit or Embedded Controller, but it is not possible to say with certainty which component it is.
Some possibility:
Is this the Original charger of the Laptop?
Defect vrms or circuits.
I am afraid this is a case for a professional repair service or the manufacturer of the Laptop.