simply put, ajax is more efficient than asp .net, am I right?
Yes, it can be used to make it more efficient. For instance if you got a webforum with lots of posts and advertisement. The, when you click 'update page' or something then the script can tell the webserver to only send recent posts, not the whole page. And then the script merges then new posts with those that are already there. This can reduce bandwith and load on the web servers/database.

Another example is webpages that take some time to load. Lets say flight ticket search engines. When you search for a ticket from one destination to another you first get a web page with an 'empty' result. But, in the background the javascript is in constant communication with the server, and as soon as a ticket is found the webpage is updated. So, instead of waiting for all tickets (one minute looking at a blank webpage), the user can see (and work) with the partial result as he is waiting for the rest to come.

- petter