The fastest person in the world is generally determined by results in the 100-meter race. Based on that, the fastest female runner in the world is either Carmelita Jeter, an American, or Shelly-Ann Fraser-Pryce, a Jamaican.
On one hand, Carmelita Jeter ran a 10.64-second race in 2009, making her the closest woman to the world record (10.49 seconds) since it was set in 1988 by American Florence Griffith-Joyner. On the other hand, Shelly-Ann Fraser-Pryce won both the 2008 and the 2012 Olympic 100-meter races, and she has a personal best of 10.70 seconds. Both women have a reasonable claim to being the fastest woman in the world.