Is BTT supposed to be using CPU suddenly like there's a rather substantial jump in CPU and energy usage when CMD is held down without anything that the key is pointing to? Meaning that the Touch Bar or nothing else uses CMD as the first key in the sequence of a shortcut. It uses about 2% when idling but then jumps to around 23% when CMD is pressed.
Depends a lot on your setup. If you have a preset installed that has some logic bound to CMD that might cause (a very temporary) spike. However in my experience activity monitor overemphasizes such super short spikes. It basically just uses the CPU for a few milliseconds until the associated logic has executed.