We calculate the average speed of a projectile in
the absence of air resistance, a quantity that is missing from the treatment of
the problem in the literature. We then show that this quantity is equal to the
time-average instantaneous speed of the projectile, but different from its
space-average instantaneous speed. It is then shown that this behavior is
shared by general motion of all particles regardless of the dimensionality of
motion and the nature of the forces involved. The equality of average speed and
time-average instantaneous speed can be useful in situations where the
calculation of one is more difficult than the other. Thus, making it more
efficient to calculate one by calculating the other.

Mohazzabi, P. and Kohneh, Z.A. (2005) Projectile Motion without Trigonometric Functions. The Physics Teacher, 43, 114-115. https://doi.org/10.1119/1.1855750

Mohazzabi, P. (2018) When Does Air Resistance Become Significant in Projectile Motion? The Physics Teacher, 56, 168-169. https://doi.org/10.1119/1.5025298