DD sounds like your son. She has a hard time getting thoughts out of her head, but I finally figured out that for her it is an issue in that she thinks in pictures so before she writes she actually has to translate the pictures into words before she can even begin to write it down. It is an extra step that slows her down.
For processing speed, the tester in our report wrote "Students with superior reasoning ability often tend to perform less well, although still adequately, on processing speed tasks." So apparently it is common, although dd did not score as low as your son on processing speed....she was still almost 3 standard deviations away from her standard scores in PRI, VCI, and FSIQ and almost 4 standard deviations from her extended scoring so it definitely is a bottle neck for her.
I would ask your tester what his or her opinion was of the discrepancy. DD's tester stated she saw a lot of perfectionism and wanting to do it correct and neatly so that she was not focused on speed and that that was very common. Good luck!
