As another data point in comparing how different editors work, here's an old one by the author of the very undervalued joe editor.
In general, there's nothing preventing screen editors from working properly and fast here, it's just not something they optimized for. If they did it at all, internal editor data structures tend to be quite simple these days, as virtual memory works for most cases. People had to get a lot more tricky on smaller devices (or they gave up early, cf. Windows Notepad).
Heretically, I'd also like to say that a lot of files that aren't binary and huge, different tools often wouldn't point at e.g. sed, as you're running into the old "Now they have two problems." issue then.