I am chiming in a bit late, but I have a daughter with a very fast processing speed. DD11 has always been told to SLOW down by all her teachers because she goes through everything so fast. Her quality of work goes down a little by her speed, but not by much (careless mistakes). We had her tested 2x, once at 6 and once at 8. We saw a dip in the processing speed at 8, but it was still high. Her only relative weakness is implied comprehension, which tends to slow her down because she does not think the way everyone else does, and hates all the poorly worded and obscure questions. In our case, it seems like my DD's high processing speed ties more to achievement. For example, she had a 160 on both math and writing on the WIAT-II, but lower on reading comprehension.
On the achievement tests at school, she will typically be at the top of the district scores on anything math or writing related because she can process those items very quickly. Writing is very quick for her because spelling is memorized, so she just has to concentrate on ideas. She has both speedy typing and handwriting skills, so that helps. Math is high because she can process information quickly and intuitively. She went through three grades in math last year in school.
Therefore, I think her processing speed really works to her advantage in school, so she gets through assignments quickly, and then has time to work on other things. The fast/accurate profile = a perceived higher intelligence and a definite advantage in school.
