1) Its user interface for fast opening of messages
2) Allowed for batch actions (like archiving multiple messages at once)
3) Added some basic offline support.
The performance is not affected by slow internet speed and it can work on very slow internet speed too.
How the technology becomes possible
from your iPhone or Android browser, archiving email is quicker because it doesn’t require a response from a remote server. Instead, we cache mail on the device itself . Actions like archiving or starring messages are first applied to this cache and then sent to Gmail servers in the background whenever a network connection is available. You only have to wait for a response from the server when you’re requesting an uncached message or list of messages. As a result, you can start-up Gmail even if you’re on a slow connection. You can even compose mail and open recently read messages while offline.”