My DS16 is also very good at math, picks up new concepts quickly and test well. But it has always taken him a lot longer to do math homework than the other kids in his gifted program making math frustrating. I didn't really understand what was going on until I got DS tested last summer. DS has much lower low processing & working memory but very gifted at Perceptual Reasoning. Therefore explaining why he picks up the material easy but gets bogged down in hours of homework. Therefore you might want a math program that doesn't include large amounts of practice.
You might be onto something here....her processing speed is quite a bit lower than her Perceptual Reasoning as well. It is hard for me to figure out how much work to "assign" to see if she has truly grasped a concept and we are OK to move on. Hopefully I will figure this out fairly quickly or we will both be struggling! We are currently refreshing Pre-Algebra so she will be ready for Algebra in the Fall.