I assume the problem is that you can't skip ahead X characters on multibyte data without parsing the bytes, because you don't know how big a character is. So you may have to read every byte.
But you can skip ahead without parsing all the bytes since UTF-8 is self-synchronizing. The only scenario I can envision is that gnu grep wants to perform unicode normalization, to catch equivalent codepoints, and that this is implemented inefficiently.
Edit: Granted, since I'm using -c, it has to look at all the bytes to find all the newlines.
