[exim] Queue_run_max question

I have a small load server (70k deliveries a day) and thought I'd try
using queue_only, with queue_run_max = 15 to see if it makes things a
bit more efficient.
I've discovered it will only spawn about 5 queue runners, and messages
will queue up for up to 15 minutes before a runner finally gets to it.
I have split_spool_directory enabled, load on the server is 0.05, disk
IO is negligible, I don't see why Exim spawns so few queue runners and
why they take so long to process messages.
It performs flawlessly when it is not in queue_only, queue time and
delivery time is ~1 second, and the only messages in the queue are
frozen or in retry state (about 100 messages on average).

Here is so relevant lines from my config, Exim 4.68, FreeBSD 6.2, Quad
Cores with 4GB RAM and 15k SAS disks.

split_spool_directory
queue_only
queue_run_max = 20

remote_max_parallel = 10
ignore_bounce_errors_after = 4d

timeout_frozen_after = 4d

delay_warning = 2h:8h:24h

smtp_accept_max = 1000
smtp_accept_max_per_connection = 4000
smtp_accept_queue_per_connection = 4000
smtp_accept_max_per_host = 200
smtp_accept_reserve = 40
smtp_reserve_hosts = obscan_hosts
message_size_limit = 50M

Any thoughts?

--
Brent Jones
brent@???