From what I have found, processing speed is tied to the physical capability of the axons/white matter. Improving blood flow and oxygenation to that area is most important. Improving functions of those axons is important so anything that can help with the health of the myelin sheath and glymph system is important.

How do you know that you have slowed processing speed ?

Some claim to have exercises that help with processing speed but others think this is more about learning to focus on a task than increasing actual processing speed. Distracted attention can slow processing speed.