Barry says that they can slow the rotation of Earth if they travel around the world at Mach 7. That's approximately 5371 MPH. The circumference of the Earth (at the equator) is approximately 25,000 miles. It would take about 4.5 hours at that speed to make one revolution. Yet, in the scenes, we see them traveling the distance in less than a second.
In order for Supergirl and Flash to cover the circumference of the Earth (24,901 miles/40074274.9 meters) as fast as they did, they would need to travel at a speed of 40074274.9 meters per second or 89,643,599.90157 miles per hour, which is much faster than Mach 7 (2,382.03 meters per second).
While Flash and Supergirl are running and flying around the globe at "mach 7" to slow down the earth, they would need to be traveling in the same direction. However, you can clearly see they are going in opposite directions during these segments.