Efficiency, Not Power
Processing speed doesn't measure how 'deep' you can think; it measures how 'fast' you can think. In an IQ test, this is usually measured with simple tasks, like finding a specific symbol in a long list or coding a series of numbers into symbols.
The Role in General Intelligence
Why does speed matter? Because the faster you can process simple information, the more 'room' you leave in your working memory for complex reasoning. If your brain is 'slow' at the basic tasks, it becomes overwhelmed when the tasks get harder.
The 'Time Pressure' Effect
People with lower processing speed often struggle with timed tests, even if they are brilliant. They may need more time to 'digest' information before they can act on it. In the real world, a slower processing speed might mean you're not the fastest at 'quick-fire' debates, but you can still reach the same high-level conclusions given enough time.
Decline with Age
Like fluid reasoning, processing speed is one of the first cognitive abilities to decline as we age. This is a normal part of the aging process, reflecting the gradual slowing of neural signals. However, for most people, this decline is offset by their increased knowledge and experience.