Dean, I understand regarding Sendmail optimization. I won't know if it's faster until my next mailing on Tuesday, but I'll post here how it turns out. Fingers crossed.
Tom, I use SMTP Server, not the Sendmail PHP setting. At least, that is what's selected in the Config so I assume that's what's being used. Host is localhost, Port is 25, reconnect every 500 emails, login ID not needed and POP checking not needed. Dean installed the system for me and it's always worked reliably, albeit slowly, and maintains the connection.
If SMTP mail is still slow I'll try internal Sendmail PHP mail setting instead, as 6 hours is not so bad as 24. But Dean says it should be taking 1 hour, so hopefully things can be improved drastically before long, somehow or other. Thanks for letting me know if you find a solution.