为什么python gevent比串行慢？

import gevent
from gevent import monkey

urls = ['https://www.djangoproject.com/','http://www.nytimes.com/','http://www.microsoft.com']
monkey.patch_all()

import urllib2
from datetime import datetime

def print_head(url):
    print ('Starting %s' % url)
    data = urllib2.urlopen(url).read()   
    print ('%s: %s bytes: %r' % (url,len(data),data[:50]))

startTime = datetime.now()
jobs = [gevent.spawn(print_head,url) for url in urls]
gevent.joinall(jobs)
totalTime = datetime.now() - startTime
print "Total time: %s" % totalTime

我的问题是上面的代码需要比串行版本更长的时间,并且在大多数情况下它会超时.这是更快的串行版本：

import urllib2
from datetime import datetime

urls = ['https://www.djangoproject.com/','http://www.microsoft.com']

def print_head(url):
    print ('Starting %s' % url)
    data = urllib2.urlopen(url).read()
    print ('%s: %s bytes: %r' % (url,data[:50]))

startTime = datetime.now()    
for url in urls:
    try:
        print_head(url)
    except:
        print 'ops EXCEPTION :('

totalTime = datetime.now() - startTime
print "Total time: %s" % totalTime