Does it? The touch bar is something like 2170x60px. That's a tiny fraction of the processing power and data bandwidth compared to an actual screen. In terms of raw pixel count, it's only about 2x as many pixels as my old Commodore 64 would push every frame. Each of those pixels is a lot deeper but it's still probably pretty much a rounding error compared to how much is being dealt with for the internal screen, never mind an external one.

(Hell, is the central GPU even running the touchbar, or is there a specialized display circuit just for it?)

That said, my reaction to this whole thread is pretty much "I guess multiple external monitors is a reason to upsell you to a Pro machine" so we agree on that point :)