Welcome to the forums, amyrps.
First, a disclaimer - I'm not a professional, but the parent of two kids who scored relatively low on the processing speed test portion of the WISC. As you've already discovered, there are many different reasons that a child might score low on the processing speed subtests. The first thing I'd note is that your child is 6 years old and doesn't seem to have any issues at the moment... so it's possible that the dip in processing speed portion of the test could be something as simple as your ds was tired or bored and not being attentive when taking that particular subtest. That said, both of my children who had relatively low processing speed scores also had real issues behind those dips in scores. My ds was first tested at 5, for a gifted program, and the evaluator noticed the dip in processing speed but thought it was nothing more than a sign of a child who liked to be very careful and thoughtful and therefore moved slowly when writing out the answers to questions (the subtest is timed). All of that is actually very true, and he wasn't struggling (or didn't seem to be struggling) in school at that time - but by the time he was in 2nd grade the undiagnosed issues behind the slow processing speed score were impacting in a hugely negative way. There are tests that can tease out the reasons for a relative dip in processing speed - when my ds' struggles at school became extreme his dr recommended a neuropscyh eval, and the neuropscyh administered not only ability and achievement tests, but also tests for adhd, fine motor abilities, etc. and was able to diagnose what the actual issue was (for ds it was dyspraxia and dysgraphia, for our dd it was a vision issue). The neuropsychologist eval was extremely helpful for us, but it's also something that isn't really necessary if there isn't an issue driving the discrepancy in scores. The first thing I'd recommend is to look at how your ds is functioning overall - do you see indications of any type of struggle or challenge? Or have any reason to believe there might be a family history of something such as dysgraphia which can have a genetic link? It can be really difficult to know at this point in a child's life, but fwiw, there are a few things you've listed which are similar for my ds: he was very slow to put on clothes etc (we just thought of him as a tortoise, and still do
... but in reality the lack of automaticity due to dyspraxia was a real challenge for him). My ds is also very slow moving and isn't terribly talkative. His dad didn't talk at all. Everyone in the family thought of this as just personality but now that there are several grandchildren who have dysgraphia/dyslexia/dysgraphia I suspect that ds' grandfather was probably also dyspraxic. My dd with the vision issues was able to learn to read but has to work really hard at it because he eyes don't track. She's been through vision therapy twice which made a huge difference but she still has to work at keeping her eyes focused (she's 16 now). When she was in 2nd grade, she started complaining about not being able to read what her teacher wrote on the board, so we took her for an eye exam and she had 20/20 vision. It wasn't until she had a neuropsych eval and scored really low on processing speed that we learned that how the eyes work together isn't typically assessed at a routine eye exam. DD has a cousin who's an adult who has strabismus that is corrected with contacts, but even so, she and dd both share a lot of the same observations about how hard they have to work at reading. You mentioned clumsiness, dd appeared to be very clumsy when she was little, before vision therapy. Both my ds and dd were suspected of having adhd when they were in early elementary, but neuropsych evals for both showed they don't have adhd, just had other things going on that had similar symptoms.
I hope some of that helps - fwiw, it sounds like you have reason to suspect there might be something meaningful behind the relatively low processing speed score.
Best wishes,
polarbear
