You noticed that your mailing is not 100% complete. You can view a graph of the "sending speed" for a mailing by going to Reports > Delivery Statistics report > click on the Completed % value for the mailing.
There are a couple primary factors that can impact sending speed-
1) The speed at which recipient servers accept mailings. Receiving servers can defer/delay or deny mailings based on various factors such as load on the server and reputation of the sender.
2) The speed at which Delivra sends mail. Our mail servers send for hundreds of clients simultaneously. Though infrequent, at times there can be delays in sending due to volume within our system.