Email time delay, while processing queue


We are sending multiple Subscribers List simultaneously from different user account (Not an Admin Account)
We are sending 20 Subscribers List, in each list there are 8000 Subscribers.
Each list taking minimum 1hour to send 8000 email campaigns

How are your phplist emails going out?
Many smtp (outgoing email) servers impose a throttle. For example Gmail paid business accounts cannot send more than 2000 per day through a single address.
Your phplist server administrator may also have set a throttle in the config.php or your upstream ISP could have placed a rate limit outgoing smtp to prevent spammers or to conserve resources.