There is a bug for 5500M GPU that causes 18W sustained power draw with one external monitor. It should normally be 5W or so.
The throttling is due to your VRM chips overheating. There is no temp sensor for this exposed to the user.
The VRM chips have no cooling. The high GPU watt draw pushes the temps of the VRMs above their thermal limits with no cooling.
The solution is to put thermal pads on your VRMs to dissipate the heat to the backplate. Completely resolved my issue.
Disabling TurboBoost can also help. Running fans at max speed at all times too because Mac is slow to ramp them up when load increases.
Apparently the 5600M doesn’t have this issue and the 5500M bug is fixed in Monterey. But the thermal pad mod is still amazing. Apple couldn’t do it stock, because there are regulations about hot a backplate can get related to risk of burns.