That link claims that ALLCAPS tend to be read letter by letter.
I find that claim to be completely false, as I am learning to read text written in Cyrillic alphabet, when I do actually read the words letter by letter and I even get surprised when I finish reading a word.
In contrast, I can read text in all caps at about the same speed as lower case text. If there's any slowdown, I say I read all caps at 95% of the speed I read text in lowercase.
Later the same page says this: "all-capital text was read 11.8 percent slower than lower case, or approximately 38 words per minute slower".
In that case that later claim completely contradicts the former claim of letter by letter reading, which is IMO about five times slower, not just a few percent points slower.
In any case, ALL CAPS text is perfectly legible, and it is actually more legible than using something like cursive, including lowercase cursive.
Hmm, I could imagine that the legibility of sentences and paragraphs, where you are pretty much reading a word at a time, is different from the legibility of things like codes where you are reading a character at a time. And for short words like table column headings, I suspect there isn't much difference either way.
