Why a message with attachments takes a lot of time to be sent?

You have to be very careful about the total size of your attachments when sending individual messages because you will send as many times that size during the delivery. That means sending a 250 KB attachment to 1,000 recipients will generate 250 MB traffic! As SMTP servers use to give around 2KB/s bandwidth per client, your delivery will take up to 35 hours to complete. There are two ways to solve this problem:

1.- Deactivate the 'Singly' option and set a group of 250 (or less). That way you will send your attachment 4 times (1,000/250). Only drawback, that mode doesn't support tags so you can't customize your message anymore. Also you may have more chances to trigger spam filters.

2.- Make your attachments available online uploading them to your web site and add links to your message so your subscribers only have to click on those links to download the files. You can then activate the 'Singly' option and again use tags to customize your message. This is actually the best method because people don't like to receive attachments directly in their mailbox. They like to have the choice to download your files or not.