In the context of a book, yes. But I mean in the context of an online Markdown editor. (Or even a Markdown editor at all. Can you actually write books in Markdown?)
Yes, you can. The "Programming Perl" book by Larry Wall was famously written entirely in "POD" format, which is just like markdown. I don't know if it was split into several files.
That's untrue. Novels are usually in the 90-110k range, depending on genre. If you're an established author, especially in fantasy or sci fi, insane doorstoppers like that do occur, but 400k is an outlier even for sff.
Here you have a list of very famous books with their word counts. Quite a few of them have more than half a million words. Thus, it is reasonable to expect that a text editor is able to handle that size. After all this is a tiny amount of data compared to what has to deal a program that does image or video processing, so there's 0 reason for a text editor to be "laggy" when dealing with just a few megabytes.
Seriously, that’s stupid. You’re collecting outliers. Unpublished writers trying to hawk 400k word novels are for the most part delusional. Every single piece in your list that goes into that territory is something written by an established author.
Though, to be fair, an author organizing a work of that size in a single markdown document has bigger problems than key lag.
A good editor can handle them without any trouble. I just downloaded Moby Dick from Project Gutenberg and opened it in Emacs. It opened instantly, I could immediately go the end (line 22333), scroll around, count the words (there are around 222617), count the ocurrences of the word 'whether' (there are 91, the last of which is on line 21769), make a change somewhere in the middle. All of this is instant.
You can do all that instantly in Vim too (... and I just checked to make sure). A megabyte of text is just not a lot of text for a good editor.