PHP interference in frames

I've got this down to a bare bones test but am still stumped. Can anyone explain why I get the behavior I do? I have a frameset with 3 frames: <html> <head><title>testing</title></head> <frameset rows='100,100,*'> <frame src='test1.php'></frame> <frame src='test2.php'></frame> <frame src='test3.php'></frame> </frameset> </html> test1.php and test2.php are both simply: <? sleep(5); ?> test3.php is simply: <? echo "This is some content"; ?> When I invoke the main frameset, the output from test3.php takes 5 seconds to appear. If I comment out either one of the sleep calls, the output from test3.php is immediate, which is what I expected in the first place. I have session.auto_start = 0 in php.ini, so this has nothing to do with sessions. I am running php 5.0.5. I get the same results with both apache 2/mod_php and lighttpd/fastcgi, the latter with 40 php processes running. I thought that each frame in a page was executed independently and asynchronously, yet frame4.php insists on "waiting" for one of the other frames to complete (if the sleeps are 5 and 10, frame3 produces its output after 5 seconds). What am I missing here?