The poor processing speed sounds like my son. He was tested at 6yo and was closer to the 79th percentile.
I recall the psychologist saying the test reflected:
1. visual processing
2. visual memory
3. fine motor skills
He has very weak fine motor skills, so he found the test very hard going and stopped before the time was up. The tester made a comment that he has near photographic memory because he took one look at the symbols, had little need to crosscheck again, and proceeded to fill in the blanks accurately.
I found Lucijane's comments very interesting, because that's exactly my son - he's usually unsure about what he sees because "it could be this-and-that". He had his vision test done a few months ago and while it's clear he has some pretty significant issues, I'm just not sure if part of the problem stems from seeing too many possibilities!