...why? I'm thinking about it and don't have the slightest idea. And it's not a difficulty I ever came across with my students when I taught ESL.
Granted, I'm not an expert in this area. I'm actually just relaying what an ESL ex of mine told me. She hated whenever she had to read things like "Calvin and Hobbes" which use all-caps, for this exact reason. Come to think of it, she was Japanese, I wonder if it has to do with growing up with a logographic writing system.
I can understand that learning separate letters for lowercase and uppercase is something that students coming from other writing systems have to learn, the same way I had to learn both uppercase and lowercase in Greek.
But it's just a year-one skill you have to learn. You learn it, fairly quickly, and then you're fine. It's not a reason to avoid using small caps. Generally speaking, by the time your English skills are good enough to read an average paragraph, text in all-uppercase has not been a problem for you for a long time. Vocabulary is the thing that takes a long time to learn, not recognizing words in uppercase.
So while I don't doubt that your ex was telling the truth about Calvin and Hobbes, I think that was just a personal annoyance of hers. It's not a widespread problem. But everybody winds up having their own idiosyncratic annoyances with foreign languages.
It distorts the shape of the words. Many people recognize many words as a whole, not by checking each letter in (eg) "the".
Yes, all-caps and lowercase have different word shapes. This is something that slightly slows down native or highly experienced speakers (readers), because they have so much exposure to reading the language that they use the word shape to help.
This is not something ESL students are doing to any appreciable degree. They have not put in the 1000's of hours of reading in the target language to read even faster by identifying word shapes. That is a level of optimization they are nowhere near.