I think you just gave yourself the answer about your child's ability in mathematics. She is unlikely to have a weakness as she enjoys advanced maths concepts. IF she simply struggles with timed tests, not the actual ability to calculate as that is a different issue, then I would hope that isn't enough to hold back a grade skip if it is otherwise appropriate and the school is otherwise willing. Battling an unwilling school or district is a different challenge again.
Read through the A Nation Deceived and A Nation Empowered. They really encompasses all the research on acceleration. Edit and select what you need including from their extensive references.
