If you're sending emails via SMTP through cpanel's Exim, make sure it doesn't add a delay for each email:
And other important thing to do is to make sure this option is disabled:
This option increases the SPAM level of your emails and adds a huge delay for each email sent.
Each message takes 5 seconds to be sent
Do you have a big file attached to your newsletter?
If you do, please make sure the option "Embed attachments" is set to "No" on the AcyMailing configuration page so instead of embedding the file within the message, AcyMailing will just add a link to this file in your Newsletter.
Do you have the option "Embed images" enabled on the AcyMailing configuration page?
If so, please disable this option... your visitor will still see your pictures inside your newsletter but instead of sending them within the e-mail, they will stay on your server.
You're using an SMTP server?
AcyMailing has to connect to this SMTP server each time you deliver a message... it should not take more than 1 second per e-mail.
Each message takes 1 second to be sent... but it's still too slow for me!
If you send via an external SMTP server, the communication with this external server takes time and sending 1 email every second is the standard rate.
If you send from your own server, you will send much much faster...