It's summer now and as soon as I plug my MacBook Pro 2017 in to charge, it starts throttling itself due to the heat of the charging battery. Same thing happened to the 2012 I had before. These things just don't have adequate cooling.
Do you have a way to monitor or measure the CPU throttling on macOS? I've found Intel Power Gadget [1] which looks like it might help, however, I'm not really sure what these graphs should look like when throttling vs not. [1]: https://software.intel.com/en-us/articles/intel-power-gadget...
On Intel chips with a 1 to 4 core boost, no cores will rise above the base clock when thermally throttled, voltage may increase, and frequencies will be lower. You can run a CPU bound benchmark until temps rise and note when the performance begins to decrease.
On desktop Intel processors, this usually happens between 70-80°C.