It's bizarre that they prioritize parsimony over correctness. Laying out long documents can happen in the background, but shouldn't be deferred to when you actually scroll.
annoyingly enough, the API has some sort of background layout support. I don't think it works well, though. I tried to use it, but it did not improve the "main thread" operations sigificantly to have an impact of fast scrolling scenario - I suspect it may not work actually