Hello,
some pages are redirected to other URL, some are blocked with robots meta tag, some have canonical meta tag pointing to another URL - all these pages are crawled, but not added in sitemap.
The crawling time itself depends on the website page generation time mainly, since it crawls the site similar to search engine bots.
For instance, if it it takes 1 second to retrieve every page, then 1000 pages will be crawled in about 16 minutes.