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.
Basically, it depends on how fast the target website is working.
Generator always starts the process from the scratch (to be able to find all changes on the website), so it will work in the same way next time.
You can improve the crawling speed using "Add directly in sitemap" and "Exclude URLs" settings.