While it does not appear that the boost ball is all that fast, Samus does clear areas at a faster rate compared to simply walking or running. So how fast is the boost ball? I thought perhaps I would attempt determining this by using Dark Samus or Gandrayda’s boost ball, since we can see the boost ball from a different perspective. I will not use Dark Samus’ version, however, since her “Boost and Jump abilities are superior to [Samus’].”  Gandrayda, on the other hand, shows no indication that her Phazon enhancement has made her boost superior to Samus’. All we know is that “Gandrayda’s ability to shape-shift has been greatly enhanced by Phazon exposure, allowing her to assume a number of deadly forms at will.” 
Since we know that Gandrayda has shape-shifted to look like Samus long before the two battle, we can safely assume that the boost ball has not been enhanced or altered in any way. The only difference is Gandrayda’s boost ball is generating her usual electricity. In the battle against Gandrayda, she will eventually shape-shift to look like Samus. Her first attack tends to be the boost ball. The distance between the two may be assumed to be 10 meters apart. It takes about 209 milliseconds to cover this distance. Using s = d/t, Gandrayda traveled 47.79 m/s, or 106.86 mi/h. As stated before with the reasons provided, this can be applied to Samus as well.