Zero means go as fast as the site allows, which is right for most of them.
This is only a floor: a host that answers 429 Too Many Requests
makes the crawl slow itself down for the rest of the run, whatever is set
here. Raise it when you already know a site is easily upset — and to
slow down one crawl rather than all of them, use Scrape it again on
that migration's review page.
Crawls are sequential, so this is paid once per page: half a second across two hundred pages is an extra minute and a half.