What is AMF? What makes flex application very fast?

BlazeDS and LCDS both support remoting over the AMF protocol.
AMF is Action Message Format.
AMF is a compact binay format, which increases the application's data
loading speed to 10 times compared with text based format protocols such as HTTP, SOAP and so on.

This AMF format is used by Flash Player for data exchange and data storage(SharedObjects).
AFM uses a compact binary format to serialize and deserialize ActionScript objects allowing you to call
and access server-side code.

This is how the Flex application is very fast compared to normal text based http and soap applications.

Flex:- BlazeDS and LCDS

We all know that Flex is developing an RIA(Rich Internet Application) application.
This flex code should be integrated with the server side code. i.e. java or php etc.
Using BlazeDS and LCDS, java and flex code can be integrated.
BlazeDS is an open source where as LCDS is costly one.
 
BlazeDS is a web application and a program that handles the all communication between java and flex application

BlazeDS enables
Invocation of remote java from a Flex application.
Translate the java objects returned from a server to corresponding ActionScript objects and vice versa.
Manages the communication channels and connection types between flex and java.
and etc.