Unexpected delay in rospy subscriber

edit

I'm trying to do some image processing in a python subscriber callback, and I'm seeing some unexpected latency in the system. It seems that even though I subscribe to a topic with queue_size=1, I still receive old messages. Also, when I remove the sleep(1.0) call from my listener callback, I get Delay: 0.019, which is the expected result.

See code snippets below:

talker.py

#!/usr/bin/env python
import roslib; roslib.load_manifest('sensor_msgs')
import rospy
from sensor_msgs.msg import Image

rospy.init_node('talker')
pub = rospy.Publisher('chatter', Image)
while not rospy.is_shutdown():
    msg = Image()
    msg.data = [0]*(640*480)
    msg.header.stamp = rospy.Time.now()
    pub.publish(msg)
    rospy.sleep(0.05)

listener.py

#!/usr/bin/env python
import roslib; roslib.load_manifest('sensor_msgs')
import rospy
from sensor_msgs.msg import Image

def imageCb(msg):
    print "Delay:%6.3f" % (rospy.Time.now() - msg.header.stamp).to_sec()
    rospy.sleep(1.0)

rospy.init_node('listener')
sub = rospy.Subscriber('chatter', Image, imageCb, queue_size=1)
rospy.spin()

When I run both nodes, listener.py outputs the following:

Delay: 0.023
Delay: 0.950
Delay: 1.879
Delay: 1.950
Delay: 2.876
Delay: 3.805
Delay: 4.733
Delay: 5.657
Delay: 5.954
Delay: 5.954
Delay: 6.882
Delay: 6.916
Delay: 6.917
Delay: 6.918
Delay: 6.916
Delay: 6.917

Is this a bug in rospy? Or, maybe there's some other Subscriber option that I should be using?

This is on Lucid/Fuerte system.

Thanks!