I imagine high processing speed also helps in building a conceptual meta structure and forming associative networks in long term memory. If you can see the forest for the trees quickly when a lot of new information is coming at you, you can categorize it appropriately (perhaps along multiple dimensions) and trigger spreading activation (that is, you can prime associative linkages between closely related concepts). With a wider associative network and more neural connections, we would expect the learning to be reinforced more frequently in long term memory for individuals with higher processing speed, potentially setting off a virtuous cycle of processing speed and encoding over time.

What is to give light must endure burning.