SOLVED by 1440p HiDPI: MacBook Pro 16" is HOT & NOISY with an external monitor!
If you connect certain GPU models (specifically the 5300M/5500M) to external monitors with certain resolutions, many users report that their MacBook gets really hot and the fans spin up really loudly.
It is documented in this nearly 200 page MacRumors thread: https://forums.macrumors.com/threads/16-is-hot-noisy-with-an-external-monitor.2211747/
Before, when connected to a 1440p monitor, my MacBook Pro 16 inch reports 18–20W of power draw from the GPU (Radeon High Side).
It doesn’t take long for all this extra power to heat up the laptop and cause the fans to spin up, even if the MacBook is idle with nothing else running!
What exactly is HiDPI mode? Well, it’s basically forcing macOS to internally render at 2x resolution. So in the case of a 2560x1440p monitor, macOS is internally rendering at 5120x2880p. You can tell, since the screenshot of the power draw monitoring utility (iStat Menus) is twice as sharp in the second example.
Now the MacBook is drawing a much more reasonable 5–6W and stays silent during normal operation, as it should! Now it feels like a Pro machine!
