I'm with polarbear. DD12 has a similar spread in reasoning/speed. Math facts quizzes were a significant part of her complete breakdown and withdrawal from school in 3rd grade.
If I had it to do over again, I would not back the teacher at all in requiring math facts drill. She still hasn't learned them at 12, and isn't likely to; her dad says he never learned them and it hasn't hurt him yet. Since you have data on processing speed, you could ask for modified assessments that let him move forward. The most important piece for us, though, has been to find conceptually interesting math to play with outside of school. Even the best 3rd grade math isn't that advanced.
My DD liked several math books: The Number Devil, Penrose the Mathematical Cat, The Further Adventures of Penrose, Math for Smarty Pants, various logic puzzles, etc. There is a puzzle called Kakuro which is like a numerical crossword, using sums as the clues, that is good for low-pressure addition practice.
It's not an easy road. Now I have a middle-schooler who loves math and STILL can't do arithmetic fluently - but her fluency in all subjects is down at the same low percentile, so I doubt we could have done much about it.
