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Mine stays warmish but not enough to make the fans consistently spin. The heat generated by using the dock is no different than what's generated by using an AC adapter with individual DP & USB cables.

This typical for laptops with discrete GPUs. Outputting video over TB can often only be done by the discrete GPU. I do not know the reason why this works this way.

On my Lenovo X1E, this is also the case. Thunderbolt video output only works via the nVidia GPU (instead of the Intel graphics on the CPU). So connecting the TB dock causes my fans to spin up, as the laptop is now rendering my desktop using a power hungry nVidia chip.

This is still noticeable in lower power devices.

I have discovered that if I leave a usb cable plugged into a MacBook Air M1 usbc socket in standby, it draws about half a watt extra, even though the cable power draw is zero.

This means the battery goes flat in a few days vs a few weeks in standby - the PHY seems to be using more power in standby than rest of laptop