I spoke with the school psychologist who administered the test, and he explained quite a bit, including his assessment that the slow processing speed isn't a concern at DS's age.
Re: coding and symbol search, DS completed 5 and 8 problems, respectively. All were correct. The psychologist indicated that his score would not have been greatly affected had he completed additional problems. I don't understand how completing additional problems correctly wouldn't have resulted in a considerably higher score.
He suggested that we request a re-test after 12 months, and if the slow processing speed persists, it may be prudent to dig deeper at that time.
I also had DS to an optometrist, whose staff indicated that he was well versed in tracking and coordination issues. After conducting a routine eye exam, with no tracking tests, he indicated that there was no concern, based on the eye exam itself, and if the slow processing speed persists beyond a year or two, we could look into visual perceptive testing. From my research, it seems that a routine eye exam wouldn't identify these types of issues. Polarbear, from your experiences, is my understanding correct? I'm not looking to chase a problem that may not exist, but I would like to rule it out without any uncertainty.