Learning how exactly downloader enqueues requests is a more advanced topic than learning how not to hit remote websites too hard. From this point of view `CONCURRENT_REQUESTS_PER_SLOT` + `SLOT_ASSIGNMENT_POLICY` are worse than `CONCURRENT_REQUESTS_PER_IP` + `CONCURRENT_REQUESTS_PER_DOMAIN`, even if they match internals better - this is what bugs me.

Indeed, there is already `CONCURRENT_REQUESTS` :)