Dunno if I agree, without knowing more.
HTML is not split into lines so your example is missing the point. Rendering html is always a difficult process because the only way to see how the text flows is to ask the text rendering engine to render it, even if the result is known to be off-screen. Then, for each line, there's several extra reflows to figure out best hyphenation position, and then you can move to the next line.
I'm not saying there are no optimisations and other tricks, but:
- this is why epub is split into chapters
- modern web browsers also lock up for considerable time reflowing html
- "threads" is not just a label you slap on a program, and in fact on single-core hardware, interruptable code is probably better than multithreaded code