I had similar results from the WISC as a child, low processing speed relative to everything else. It's been something I've been meaning to ask about for a while as well, particularly since one educational professional predicted my test scores (roughly, of course) from certain problematic behavioural patterns, which was enough evidence that there's something meaningful there to get my attention.

My memory of the tests isn't entirely clear, but one task was something like transcribing unfamiliar symbols according to a substitution key in a particular time span. If that's similar to Daniel's experience, then any advice that cognitive science types can come up with here could be useful to both of us.

ETA:

I think this study details the task I remember.