I'm glad her teachers are taking the time to help her slow down this habit.
At home, I would put emphasis on her work's quality, not quantity. She could work on some kid-size household tasks that require attention to detail and not speed to finish. Craft work -- usually a hit with nine-year-old girls -- requires that directions be followed, and will also give her practice at slowing down. Model-building and cooking with recipes are also suggestions I make when encouraging young gifted kids to learn the value of following a process. Make it a point to acknowledge some great effort -- artwork, a building, a hand-crafted item, etc. -- that took some time and care to create.
These ideas, plus the fact that after third grade, work becomes less rote and requires more attention, may help her to take her time. Good luck.