Some argue that mixed case texts are more readable because the ascenders and descenders provide "synchronization points" for our eyes in scanning text. Also (and less controversially) uppercase is larger so it is slower to read it, because there are more pauses to move your eyes to the beginning of the next line.

There are plenty of languages without upper and lowercase, I've never heard of them being slower to read.

Yes, that's why the first point is more controversial. But the second is interesting: alphabets that lack lowercase, like Korean hangul, have bigger characters but they pack more information into each of them.

thanks for this. but c.f., javascript vs python syntax

Lower-case letters evolved from handwritten Roman letters. Upper-case is exactly carved Roman letters. Mixed-case comes from initials — https://en.wikipedia.org/wiki/Initial

they are way easier to read than all uppercase.

since the responses have mostly been literal (no pun intended) perhaps i should clarify, i meant "case" in general