USB C hub temperature
As usual, the flood of different converters uses only a few board designs, and the most popular one seems to use cheap components, most likely for power conversion from 5V to 3.3V for its internal electronics. A step-down converter (similar to a switching power supply) costs more than converting the excess voltage to heat via resistors, after all.

I have now found a product made by iTeknic (model # IK-UC001) which actually carries a warning that it might heat up to 130 F in use, but so far it has stayed cool in tasks where the other adapters would heat up close to those 130 F. I have not looked inside (yet) but suspect it uses a step-down converter. They cost about 0.1% of the hub's purchase price, after all. 

I guess the Networkx hub's external power supply (for a device meant for portable use!) should had been a warning. The iTeknic hub comes without such a power supply.