notify delays using nsupdate

[Zain Al Abdeen]

Hi,

I was testing notify on test boxes configured with just a couple of 
zones. There is no load on either of the boxes acting as master and 
slave servers respectively. Both server are running bind 8.4.1-REL.

When a zone file is update manually on the master server and ndc reload 
<zone name> is issued, notify is send in almost 20secs and the slave 
runs an AXFR almost immediately.

When a zone is updated using nsupdate on the master server, notify is 
issued in 2-10 mins and the slave runs an AXFR immediately. Whereas the 
master server is responding immediately with the updated SOA after nsupdate.

My questions are;
Why there is delay in sending notify when using nsupdate ?
Can the named be tuned to send notifies immediately after a successful 
nsupdate ?
What are implications for implementing nsupdate along with notify in an 
environment where there are 25,000 plus zones and more than 5 slaves to 
be notified ?
Can using bind 9 or higher version of bind 8 improve the above mentioned 
scenario ?

Thanks & Regards,
Zain