The brain's processing speed is surprisingly slow. Despite receiving a massive amount of sensory input, the brain processes information at only about 10 bits per second. This is millions of times slower than the input rate. The slow speed explains why we can only focus on one thought at a time. Our brains evolved to handle a slower-paced world, and we're not equipped to process multiple complex thoughts simultaneously. It's a limitation that likely originated in early animals whose primary need was navigation, which required following one path at a time.