I think that brain one was from a game of telephone with the real fact that a large portion of our brain is dedicated to image processing and object identification. Another portion would be dedicated to sound recognition with a decent amount of circuitry going into the recognition and parsing of speech. Memory will also take up some of the capacity as well as mapping desired actions to sequences of signals for muscle activation. After all the things our brains need to do just to accomplish all these things we take for granted are accounted for, it doesn't leave much capacity left over for thought.

Though, at least in my experience, the most powerful analysis the brain can do is in the subconscious. So many times I've faced a difficult problem where I've been unable to make any progress, take a break, then later return to a much easier problem. Or even with skill development, try doing something too hard for a bit, then sleep on it and try again the next day and it might suddenly be easier. This works best for dexterity skills, I've noticed it a lot in Beat Saber.

So it's like you can take whatever was left over from the first paragraph, then take a small amount of that and that's your conscious thought capacity and the rest is given to subconscious processing.