Email Sending Delays

We are currently using centos-webpanel.com for sending our emails. We have recently had a spike of Magento2 / system generated emails from 300+ per day to 2200. Some Customers complain that emails are delayed from 15 minutes to 8 hours in some cases. We want to know why this is happenening and what we can do to ensure our eMails get sent out as fast as possible.