One of the issues is that processing speed is, like many other clusters, a symptom, and not a single specific etiology. We can call it processing speed, but what it really is is how efficiently an individual can complete a rote visual-motor/fine-motor task. Many, many factors can reduce performance on this task (I've listed a few of them already, and others have come up with additional plausible explanations).
As with many cognitive profile discussions, it is worth re-centering on the presence or absence of pathology, which, at it's most basic, is simply the question, "does anything about this interfere with being a functional, independent human being, living a happy, satisfying life, or attaining personally-significant goals?" If not, enjoy the diversity of the human experience, and leave it at that.
I could also throw out a few more thoughts about mapping to long-term memory, retrieval fluency, meaningful vs symbolic content, motor sequencing, etc., but they're still only important to life if the answer to the above question is 'yes' to any part.
