Right- but if you have a long line that is, for example, a JSON object, then surely it can't be properly be validated or syntax-highlighted before the entire line is scanned?

I do agree that Emacs can be slower than the terminal when handling long lines/files, although (depending on your case) this can be easily mitigated by running a terminal inside of Emacs.

Generally though, for everyday use, Emacs feels a lot snappier than VSCode.