Scrapy Concurrent Requests

This is my code snippet

import scrapy
class QuotesSpider(scrapy.Spider):
    name = 'quotes'
    custom_settings = {
        'CONCURRENT_REQUESTS': 25,
        'CONCURRENT_REQUESTS_PER_DOMAIN': 100,
        'DOWNLOAD_DELAY': 0
    }

    f = open("list.txt")
    start_urls = [url.strip() for url in f.readlines()]
    f.close()

    def parse(self, response):
        for quote in response.xpath("//div[@class='border block']"):
            urlgem = quote.xpath(".//div[@class='col-md-4 pull-right']/a/@href").extract()
            if urlgem:
                yield {
                    'text': urlgem,
                }

Using Terminal I execute above code using command

scrapy runspider quotes_spider.py -o quotes.json

list.txt contains 50 URL of same domain separated by lines

As per my understanding the code should put 25 requests to the domain for 25 URL (from a list of 50 URL) and should complete in 2-3 seconds time span and generate file name quotes.json

The output in file quotes.json is coming out as expected but SCRAPY is not performing the task concurrently instead it is fetching URL one by one and takes approximately 55-60 seconds to complete.

Please help!!!