How "download_slot" works within scrapy

I'v created a script in scrapy to parse the author name of different posts from it's landing page and then pass it to the parse_page method using meta keyword in order to print the post content along with the author name at the same time.

I've used download_slot within meta keyword which allegedly maskes the script run faster. Although it is not necessary to comply with the logic I tried to apply here, I would like to stick to it only to understand how download_slot works within any script and why. I searched a lot to know more about download_slot but I end up some links like this one.

An example usage of download_slot (I'm not quite sure about it though):

from scrapy.crawler import CrawlerProcess
from scrapy import Request
import scrapy

class ConventionSpider(scrapy.Spider):
    name = 'stackoverflow'
    start_urls = ['https://stackoverflow.com/questions/tagged/web-scraping']

    def parse(self,response):
        for link in response.css('.summary'):
            name = link.css('.user-details a::text').extract_first()
            url = link.css('.question-hyperlink::attr(href)').extract_first()
            nurl = response.urljoin(url)
            yield Request(nurl,callback=self.parse_page,meta={'item':name,"download_slot":name})

    def parse_page(self,response):
        elem = response.meta.get("item")
        post = ' '.join([item for item in response.css("#question .post-text p::text").extract()])
        yield {'Name':elem,'Main_Content':post}

if __name__ == "__main__":
    process = CrawlerProcess({
        'USER_AGENT': 'Mozilla/5.0',
    })
    process.crawl(ConventionSpider)
    process.start()

The above script runs flawlessly.

My question: how download_slot works within scrapy?

from How "download_slot" works within scrapy