You noticed that your sent email is not 100% complete. You can view a graph of the "sending speed" for an email campaign by going to Analytics > Campaigns > Delivery Statistics report > click on the Completed % value for the campaign.
There are a couple primary factors that can impact sending speed-
1) The speed at which recipient servers accept the sent email. Receiving servers can defer/delay or deny emails based on various factors such as load on the server and reputation of the sender.
2) The speed at which our software 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.