[thatswrong0]

Yeah this this has been abysmal for me. It does this even if you use it with the main display closed (clamshell mode)

Makes my 2018 "Pro" machine feel like an 8 year old device struggling to keep up with very basic tasks because of thermal throttling (and this is even after getting the fans cleaned by Apple recently)

[eyelidlessness]

Clamshell mode is generally worse for thermals. Even though you’re not dedicating power to the internal screen, you’re trapping more heat in the case. Other than that, take a look at my sibling comment for a potential solution.