Question

Why speed is called a derived quantity?

Answer 1

Speed can be given as distance travelled / time taken and these both quantities are fundamental quanties and hence as speed is derived from these both quantities it is known as a derived quantity.

Answer 2

The speed is a derived quantity as it being derived from a defined set of quantities through a formula. There are only 7 fundamental quantities which are being defined with their individual fundamental units. Of those, length and time are two of them.

These two when taken in ratio, derives a quantity as a measure of length in a specified time parameter known as the speed.  

$\text{Speed} =\frac{\text {Distance}}{\text {Time}}$

Thus, it is a derived quantity.