In general most of the “embeds” are embedded into the post HTML and not fetched at reading time. As far as images, those are handled by the browser as with any other website and do unfortunately affect the reading position. Youtube is not loaded by default, it is actually static and only embedded once you click it.
Discourse doesn’t load everything, in fact it only loads X number (which I think is 25-50 depending on settings and if you’re mobile). So if you were to go to a post number in the middle of a thread, you’d only load X number of posts before after. As you scroll down or navigate in some way does it load more in either direction. You can easily get to the bottom or top with the little post count box in the lower right.
There isn’t a way to disable the Onebox (it is what they call it) unless I were to disable it for everything.
[spoiler] is so hard to type, they changed the button stuff and I haven’t taken the time to update my code to handle it.
Why would it be removed?