Python 脚本跑着就不跑了,求大佬帮忙解决

写了一个检测网站服务器的脚本跑着跑着就不跑了,刚开始线程多大跑多快!到后来就会越来越慢,在到最后直接不跑了!怎么能解决?谢谢大佬们!代码如下

#!/usr/bin/python3
import requests
import time
from multiprocessing import Pool
def server(url):
    try:
        url1=url.strip()
        res=requests.get(url=url1)
        res2 = (res.headers['Server'])
        if "Apache" in res2:
            print("该网址是 Apache:{0}".format(url.strip()))
            f=open("apache.txt","a")
            f.write(url.strip()+"\n")
            f.close()
        else:
            print("其他站点")
            fx=open("noapache.txt","a")
            fx.write(url.strip()+"\n")
            fx.close()            
    except:
        print("网络")
        fb=open("no1.txt","a")
        fb.write(url.strip()+"\n")
        fb.close()
if __name__=="__main__":
    urls = ['{}'.format(str(i)) for i in open("urls.txt")]
    pool = Pool(processes=20)
    pool.map(server, urls)