Delivery rate drops soon after restart

[Ram]

I have a postfix (2.7)  server where as soon as I restart the mails are 
moved rapidly from incoming queue to active

But soon ( after 5 minutes )  the queue manager is left behind  .. the 
incoming queue keeps increasing 10k+  but active queue has hardly 10-15 
mails
 After checking 
http://www.postfix.org/QSHAPE_README.html#incoming_queue  .. I have 
increased in_flow_delay to 10s
But that does not help.

I wonder what happens when I restart postfix that the queue clearing is 
so fast for the first 5 minutes or so




Thanks
Ram