Dynamic DNS record expiration, journal file behavior

[Joe]

I'm running Bind 9 and have some questions about the behavior of
Dynamic DNS.

I understand that when a DDNS update is sent the entry is placed in a
journal file (.jnl) which is not added to the zone file for up to 15
minutes.
(http://www.bind9.net/manual/bind/9.3.1/Bv9ARM.ch04.html#journal).

Is there a way to shorten this update time? I would perfer if the
updates are next to immediate. I realize the number of updates/second,
hardware speed, etc. may be a factor.

Also, I know that when the TTL value in a DDNS record expires a device
will query the server for an updated record... but what happens on the
server side to a DDNS record that has an expired TTL? Is the record
dropped from the zone or is it kept there until the record is deleted
or updated by the DDNS client?

My goal is to have DDNS updates appear in the zone almost immediately
and have a very low TTL to ensure querying entities always have the
latest IP information. This is a network running internally.

Thanks!