I've come to understand how processing speed works on a deeper level and my interpretation is that processing speed is the speed with which we can make basic information useful to us.

A way to explain this is to give someone a number, say 1. This is a basic piece of information and its easy to interpret. How quickly did you make sense of the 1?

Now the spread would be that all information requires making sense of before we can use it and I have found that I have an excellent spatial working memory but slow "coding" or visual processing speed.

This means that I can take huge stretches of information but it takes time for me before that information becomes understandable.
This shows up in the behavior of necessary contemplation afterwards in which I go through my impressions during the day and use that information to learn.

This explanation also sheds some light as to how people can have high verbal comprehension and performance index while not having good processing speed. Processing speed if low hampers performance index to some degree but less when it comes to verbal skills.

Some thoughts of mine.. I've had periods where my visual processing speed has been extremely high which during the time made me able to react much faster to people and my surrounding.
I think that having a balanced profile allows us to find our level of function easier but mine fluctuate a lot!