And I may be in the minority on this, but I think that the WISC-IV tests for processing speed and working memory are not the last word on speed and memory skills.
I'll agree with you on that one. My dd12 probably does truly process much slower than her other abilities, but I suspect that my dd10's PSI score came out lower b/c she made a ton of mistakes. Attention to detail is not her strength. I wasn't told that was the cause for her scores (it's just my suspicion), but she really is a pretty fast kid who is highly error prone.