i) Keep your mailing lists as clean as possible
Remove all invalid and unsubscribed email addresses from your mailing list. Sending to these email addresses will create many bounce backs and will result in slow in email delivery. In the long run, it will lead to your IPs getting blacklisted too.
ii) Size of email
Keep your email as simple as possible. Good email content does not need attachments at all. If you have images and/or attachments, it is advisable to host them externally first and then provide a link for the receiver to refer.
iii) Schedule your email campaign
If you are sending high volume of emails, make sure you can manage your sending time properly. It is advisable to schedule your next email campaign 1-3 days after the first email campaign. This is to ensure all your emails are sent out and all bounce back emails are captured.
iv) Internet connection
Check the Internet speed at your home or office as it will affect the delivery speed as well.