[thaumasiotes]

Lawyers were mentioned directly in your grandparent comment. Sales and athletics also easily show at least that much variation from person to person. Business owners show much, much more variation than that, even if you restrict the field to very similar businesses. Researchers show variation similar to business owners (i.e. mind-bogglingly high).

You can easily construct differences of larger magnitude for what you might think of as "simple" jobs like telephone receptionist; a fluent english speaker is going to be more than 10x as productive in that job, assuming they're supposed to do business in english, than someone who can barely get by, and probably billions of times more productive than someone who can't speak english at all. There are a lot of Chinese people out there whose highest ambition is to "work in an office". I've met some of them! Most will never achieve that goal, because they don't have the requisite skills.

Most people who don't speak english won't apply for a job answering a telephone in english (but a lot of them will! This is exactly the kind of customer service representative everyone has grown to hate). I'd like to see some software job openings that specified a skill set, and were willing to hire people with that skill set. That doesn't seem to be the direction people are headed in, though...