This is like asking for a source for common sense statements.

HDDs are pretty terrible at random IO, which is what reading many small files tends to be. This is because they have a literal mechanical arm with a tiny magnet on the end that needs to move around to read sectors on a spinning platter. The physical limitations of how quickly the read right head can traverse limits it’s random I/O capabilities.

This makes hard drives, abysmal, at random I/O. And why defragmenting is a thing.

This is common knowledge for anyone in it and easy knowledge to obtain by reading a Wikipedia page.

SSDs are great at random I/O. They do not have physical components that need to move in order to read from random locations they generally perform equally as well from reading any location. Meaning their random I/O capabilities are significantly better.