Paul,
100,000 in 24 hours is quite slow in itself. Your now 4-5 day delivery time is simply unbearable.
CPanel in its default configuration (prior to upgrade?) should have accepted all of your emails in a reasonable amount of time. Something like a maximum of 3 hours to send 100,000 messages to the server would be a generous estimate. After that, it's up to the SMTP software (Exim, in CPanel's case) to deliver each message in queue.
It's possible that your host is running a very large number of users on the server who are also sending out email or making extensive use of MySQL. This is the most common cause of delays.
Unfortunately, there is no way for me to know what the CPanel upgrade did - I will have to look into it. I have a large number of clients on CPanel and have access to a dedicated server running the software, too, so I should be able to come up with a solution if the slowdown is indeed caused by the upgrade.
Another thing known to slowdown mailings to larger lists is lack of a Primary Key on the "id" field of the lm_sendq table.
With PhpMyAdmin, which is more than likely accessible from your control panel, open up the lm_sendq table and hit the button next to the "id" field that shows "Primary Key".
This change is usually only necessary on much larger lists but could help somewhat in your case. It will be implemented automatically on all installations in the next update.
I will see what I can determine with the admin access I have to a CPanel dedicated server. On second thought, though, we have applied a patch to Exim and prevented it from being updated during CPanel updates so it's hard to say if I will be able to reproduce these results. :?
