GMail labs Background Send

Gmail team released a labs feature, Background Send to send emails faster in the background without waiting for Gmail to send the message, you can enable this feature from Labs tab in settings to save couple of seconds or more time that matters for you.

Enabled Background Send in Settings

After enabling Background Send feature, you can continue to read messages in inbox, compose new messages, chat with people and by hitting send button Gmail quietly sends one or more messages in the background. [Via]