A kid with sloooooow processing speed isn't going to get great scores on speed Math no matter what school you put him in. The difference is that in a speed-Math school the child will be held down conceptually to the level of his Processing speed, while a school thei the concept-Math orientation will allow a pokey kid with High Understanding to move ahead.
My sweet 'n of 1' has always been in speed=math accomplishment schools and has always been about one standard deviation below average. I don't think he would have improved without practice, and he has practiced quite a lot, however is his always about one SD below average for his age - the other kids always improve about the same amount that he does! So DS is improving, and still 'slow' relative to his agemates. If my son had been in an environment which didn't put any emphasis on speed, he might have practiced less and fallen even more behind, but I think that he might have been a happier kid because he was allowed to do more interesting Math at a younger age.
I hope that clarifies, because I didn't mean to imply that speed can't improve. I'd rather hothouse speed at home and have a good 'thinking level' match at school than vise versa, but that might be just me.
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