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.

An example of this situation would be copy (from terminal emulator) and paste to editor window the output from poudriere to see specific ports are included in skipped (or succeeded) ones on building a large number of ports. Built, ignored, failed, skipped ports has single output line respectively.

In this specific cases, editors/leafpad (and IIRC, editors/pluma, too) locks up on horizontal scrolling (in case turned over at right end of window, vertical, too) or searching. devel/geany works fine.
Just examples I've tried before.