The process is not sped up, but the user experience is; which is of course a plus.
Video embedded below.
"We’re always looking for ways to make Gmail faster. One of the most common delays happens after you hit that “Send” button, when you’re waiting patiently for a couple seconds for Gmail to send your message. If you send a lot of email, that can add up to a lot of lost time.
To help give you that time back, there’s a new feature in Gmail Labs called Background Send. Once you turn it on from the Labs tab in Settings, you can get on with what you’re doing while Gmail quietly sends off your mail in the background. You can keep reading your inbox, compose new messages, chat with people — all the things you’d usually do. You can even send more than one message in the background at the same time."