Who writes those premade robust libraries? Who improves them, when a few percentages of improvement will be multiplied by enough machines to come out to millions of dollars saved?

If you have a pile of raw iron and you're currently running your own smelt for cost savings over the other remodelers, maybe even though you are in the remodeling business you could use those skills.

I agree though that contracting is by far the best way to grade performance. But the Google style interview is the only reasonable alternative I know of to filter out people who don't actually know the technologies they are talking about and can fake it with other people's code for months.