There are many factors that you need to consider when you are sending high volume emails and below are few general factors:
i) Email size
The size depends on the quantity of information in the email and it will slow down email delivery if your email size is big. For example, your email contents of attachments or graphics such as pictures. We may suggest you to use links to files hosted on a web server.
ii) Invalid email addresses
If there are many bounce back emails due to invalid email addresses or emails rejected by subscribers - which is not double opt-in email addresses, it will create email queue and eventually slow down the delivery speed.
Sometimes, the use of an anti-virus program can influence email delivery speed, because it takes a few seconds to scan each message before it will be sent out.
iv) Transmission speed
If the speed of your Internet connection is slow, it will influence your email sending experience too.