Why Are Capital Letters Difficult?

Why are words written in capitals more difficult to read than words in small letters? There are two basic reasons:

Firstly, at least for native English speakers, children usually learn to read and write small letters before capital letters.
Secondly, and more importantly, words written in capital letters have no "shape". Words with small letters go up and down. Some small letters have "ascenders" (like the letter b). They go up. Some small letters have "descenders" (like the letter p). They go down. Some small letters have no ascender or descender. They stay in the middle. So small letters vary in height. But all capital letters are the same height (BP).

When we read text, especially when we read fast, we do not read each individual letter. Instead, we read whole words and phrases. And we recognize these words and phrases partly by their shape.