Question: Because of certain reasons, I'm sometimes tied to an extremely slow internet connection. What I realized with this, is that certain sites (for example, any subdomain of blog.hu) works in a way that needs a certain part of the site be loaded before comments are loaded too. I assume it's because of an AJAX operation, as the comments themselves are not visible within the HTML file.

In contrast, forum engines, such as vBulletin are often free from AJAX, meaning that when a thread's HTML is loaded, every comment in it are "hardcoded", meaning that even if the "fancy parts" (CSS, images and so on) are not loaded, I can already read the comments.

My needs are very special though, so I'm interested - is there any general UX advantage of any of these?

Answer: 
There are physical limits as to how long a line of text can be before it gets difficult to process. Much like breathing while speaking, the mind needs periodic breaks to process the string of letters and numbers it has just taken in. Line breaks provide an opportunity for this to occur. A general rule that I have heard is to aim for roughly 80 characters per line.

However, this is not the whole story. It seems like we could simply base our font size off of the vw unit to ensure that we always hit this magical 80 character ideal, but no one really does this. Why not? The short answer is peripheral vision. Keep in mind that our eyes and brain also need to keep track of what line we're on, so when we jump after the line break, we know where to continue from. If the text block takes up too much of our field of view, the start of the line will be too far into our peripheral vision by the time we get to the end, and we lose sight of our place. That's no good either.

The best way to solve the demands of both character limit and field of view limit is to just enforce a maximum width that (hopefully) ensures we're fitting 80ish characters to a line while those characters remain large enough to read easily and the entire block remains small enough to stay within readable focus.

A couple of points about outlook and gmail specifically. Outlook uses some of the excess horizontal space to display both navigation and the list of messages in left aligned panes. The remainder of the window *appears* to be used for the message body, but extreme widths (using multiple monitors) reveal that it quietly enforces a max-width for the body contents. Gmail surprisingly does allow body text to grow effectively infinitely. I pulled up a couple of wordy emails to check, and to be honest long paragraphs don't read especially well. Note that the *compose* dialog is width limited, so it's somewhat surprising that they aren't limiting the width for reading. Perhaps they've found in their testing that emails are typically fairly short or (in the case of promotional) have already been laid out by a designer. Or perhaps Google needs to update their desktop layout...
@JoshDoebbert has a good point on the text legibility. [This other question](https://ux.stackexchange.com/questions/34120/relationship-between-font-size-and-width-of-container) has some interesting information on the topic.

I just wanted to add that percentages are good to let the content adapt to smaller screen size. As you said there is a minimum size and in the same way there is maximum size that lets the content be understood correctly. Whether it is text or images **the screen size in both height and width should be considered to display the content correctly**, and set relevant breakpoints for both parameters.

[![enter image description here](https://i.stack.imgur.com/oTPat.jpg)](https://i.stack.imgur.com/oTPat.jpg)