The long-line issue is real, though my statement was specifically about building UIs with widgets/overlays/text properties - not handling arbitrary files. In that context, Emacs's display engine is genuinely well-optimized: it handles overlays, faces, text properties, and redisplay regions efficiently.

When you're building a UI, you control the content. Lines are short by design (form fields, buttons, lists). The pathological case of a 50KB minified JSON line simply doesn't occur.

The long-line problem stems from how Emacs calculates display width for bidirectional text and variable-pitch fonts - it needs to scan the entire line. That's orthogonal to rendering widgets or interactive buffers.