I hate to sound like a broken record on this one, but I have to refer to that Developing Math Talent book by Susan Assouline. This issue is one of the myths and issues discussed at length in the book. GT kids are NOT necessarily good at memorization of math facts. Mathematically gifted kids want concepts. They want to see the connections, the big picture and the patterns. How much is left when you take 6 away from 13 is just not interesting!!!!!

Many mathematically gifted kids are horrible at timed tests in math. It isn't math. It's memorization. Use some of the links from Hoagies' math pages to find the quick tricks and patterns for learning patterns in math facts. There's links to sites that help make memorization more fun, especially for multiplication.
http://www.hoagiesgifted.org/math.htm

Here's a nice article that explains why it *is* important to eventually memorize math facts:
http://www.hoagiesgifted.org/why_memorize.htm

But most GT kids need to be allowed to move on while still working on those math facts. My son is working 3 grade levels above his chronological age/grade level in math but still a little slow with math facts. Yet he was relishing his homework last night as he worked with mixed fractions, simple algebraic equations and probability questions. Don't hold them back on the math facts. Just keep working on them as you move along.

And, BTW, one reason my son did horribly on the timed tests at first was because he kept stopping to erase and have his answers written perfectly - not because he'd gotten it wrong! Once we told him to just write and go on to the next one he got much faster!