scrapy piplines下载图片速度很慢怎么优化，,没有用文档中所说的图片管

scrapy piplines下载图片速度很慢怎么优化，,没有用文档中所说的图片管

没有用文档中所说的图片管道来实现，为什么我这么实现就很慢呢？

以下附上代码：

# -*- coding: utf-8 -*-# Define your item pipelines here## Don't forget to add your pipeline to the ITEM_PIPELINES setting# See: http://doc.scrapy.org/en/latest/topics/item-pipeline.htmlimport osimport urllib2import requestsclass boboCrawlPipeline(object):    def __init__(self):        self.f = open('data.txt', 'w+')        self.browse_headers ={'User-Agent':'Mozilla/5.0 (Windows NT 5.1; rv:22.0) Gecko/20100101 Firefox/22.0'}        if not os.path.exists('av'):            os.makedirs('av')        os.chdir('av')    def process_item(self, item, spider):        title = item['headTitle'][0].split('-')[0].encode('gbk').rstrip()        dirname = title.decode('gbk')        if not os.path.exists(dirname):            os.makedirs(dirname)        self.f.write(title+'\n')        for img in set(item['imgurl']):            self.down_link(img, dirname + '/' + os.path.basename(img))            #self.f.write(img+'\n')        return item    def down_link(self,url, filename, istorrent = 0):        forumurl = "http://38.103.161.185"        if os.path.exists(filename) and os.path.getsize(filename) > 0: #TODO MD5            return        if url.find('attachments/month')>=0: #如果是本论坛的图片则补全地址            url = forumurl + "/forum/" + url        elif url.find('attachments/day')>=0: #如果是本论坛的图片则补全地址            url = forumurl + "/forum/" + url        #print("+++++++%s+++"%url)        attempts = 0        while attempts < 10:            try:                req = requests.Session()#新建连接来下载图片                save_html = req.get(url,headers=self.browse_headers,timeout=10)                if save_html.content == None:                    return                f=open(filename, "wb").write(save_html.content)                f.close()                break            except Exception as e:                attempts += 1                #self.log(e)                    #self.log(filename +"||"+ url)          return    def close_spider(self,spider):        self.f.close()

编橙之家文章，