I'm not sure what's happening, but I am sure it isn't new. I had to learn the hard way 15 years ago that the average American adult cannot parse a full-page email in standard English. It seemed crazy to me at the time and seems crazy to me now, given that the average adult has completed elementary school, but most people are barely functionally literate at all. I don't expect you to believe me. It's a weird claim. Bu…
Of course.
I would likely find this situation very unsettling, if not stressful. I would probably be caught off guard. I would probably perform badly if I accept at all for all sorts of reasons unrelated to my ability to read smoothly. Unless I'm feeling particularly positive and gather the energy to pace my mind, apply myself, enter a character like I'm having a role in a play, and remember to be slow, and to forget about the content. Reading out loud is more complicated than simply reading for yourself: you are, at the same time, both reading and articulating speech.
What's more, when you speak, you are using your own words, your own (oral) phrasing, and you know where you are going, or at least enough to have a decent prosody. When you read out loud, you are reading phrasing from someone else, which may make the prosody less smooth. And if you haven't read the whole sentence in advance, or at least ahead enough, you may struggle.
Independently of knowing how to read (fast), reading out loud probably takes practice to be smooth.
With your experiment, you are testing all sort of things unrelated to knowing how to read, to the point that you can't draw any conclusion on the ability of people to read.
Parsing a text also depends on the writer's ability to write well. If the text is boring or its phrasing is overly complicated, yeah, it will be difficult to read.
We also live in a world plagued by focus disruptions. You are not only dealing with people's ability to read, but also their ability to remain focused… in a setting where they possibly get interrupted very frequently.
For all sorts of reasons, reading a wall of text is indeed hard. This includes the reader's environment, the presence of their smartphone next to them, how tired they are, whether they are concerned by something else, whether the text is actually interesting to them, and how well the author of the text writes.