Exceeding the capacity of your working memory1 is one of the surest ways to put an upper-bound on your performance in any(?) domain.
Your working memory represents the data that you have at-hand at any given moment, and if you need to be considering more data than you have capacity for you’ll start incurring costs from swapping information into and out of working memory. It’s not that it’s impossible, but the difficulty curve gets steep the more you exceed your capacity. The swapping cost here is likely a major contributor to the fatigue of context-switching.
Since working memory is so very limited, moving as much as you can out of conscious consideration in your primary domain is a huge performance gain. This is why it’s so important to Learn your tools: any time you need to consciously consider how to accomplish an action mechanically, you’re losing a working memory slot that could be occupied by higher-level concerns.
Further Reading
Working Memory and Intelligence: The Same or Different Constructs?. Ackerman, Beier, and Boyle
Footnotes
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5-9 pieces of information, per George A. Miller ↩