I don’t know that I completely agree. It depends on the functionality offered, right? Like vim, for example, can struggle with very large files if you ask it to do syntax highlighting all the way from the beginning (or, it can give you syntax highlighting that is just wrong if you don’t). I don’t think vim is very inefficient (could be wrong there, though), and I don’t see any way to generally do syntax highlighting without looking at the whole file (although, of course, in practice there are often shortcuts for specific languages…)