AimIsADickIn Esperanto, functional illiteracy is unheard of, because of the easiness of our language.
I have not really read the rest of this thread (given its length) but I would like to comment that I would be very very surprised if this wasn't largely or entirey driven by selection bias/confounding.
Also would like to comment that if you can prove or show evidence that simplicity/consistency/regularity of syntactic structure of a particular language leads to differences in processing speed compared to a language that is more "complex" (in the sense of syntactic structure, irregularities in morphology/conjugation that type of stuff, ambiguities in semantics), to quote my undergrad advisor, "I will give you a PhD". To my knowledge that is not a settled point. Not sure if there will ever be a study on that tbh, seems extremely difficult to set up that sort of experiment in a good way (want native bilingual speakers of a "simple" and "complex" language, doubt there's that many of those out there and linguistics depts are struggling for funding as is). If only "wild child" experiments weren't looked down by IRB, would love to see if my woodland child could flex esperanto twice as fast as english.