Hi Momoflotsofkids and welcome here,
Processing Speed index, is a combination of the Coding and Symbol Search tests.
These are basically a test of Visual and Motor skills.
For example with the Symbol Search?
On one page their is random arrangement of different symbols.
He would be shown some symbols, one by one.
Then have to have point at those symbols on the other page.
Where it basically involves holding an image of the symbol in his mind, as he looks for and points at them.
So that problems with this test, can first be a difficulty with holding a visual image of the symbol in his mind, as he looks for it?
You can probably imagine the difficulty with this, if he can't remember the symbol that he is looking for?
But the other possible problem with this test, is a difficulty with fine motor skills.
Which can make it difficult to point at, or cross out the symbol with a pencil.
With the Coding test?
I've attached an example of it.
In the top row, you'll see the numbers 1 to 5, with symbols under each number.
Then below it is the test, with random numbers.
Where the right symbol has to be drawn under each number.
Which is a timed test.
So for example, when he sees a 2 in the lines below?
Then he has to hold an image of the ] symbol in his mind, as he draws it under the 2.
Where difficulties with visualizing the symbol, and/or drawing the symbol?
Will cause problems with this test.
So that basically, difficulties with holding a mental image, and being able to draw the mental image?
Are what are behind Processing Speed.
Though the Block Design test, also involves holding a mental image of a 'pattern of colored blocks', and then making this same pattern.
So that you could look at whether he has a difficulty with holding a mental image of the pattern in his mind?
Or if he has fine motor difficulties, that caused a problem with the tests?
Or perhaps both?