Our Dr. said that processing speed can be improved in an particular area by (lots of) practice. Example: multiplication facts.
This will not transfer over to other areas however; those would have to be specifically practiced as well.
As far as my own processing speed issues, I actually do think that I have caught up considerably. Maybe it is only in appearance however; I can get to correct answers faster than most other folks because I deal with complex problems that require understanding and keeping in mind a lot of factors, and I have better logic skills.
I still cannot beat anyone (except my kids) time-wise at multiplication tables (but then again I don't practice).
