Daniel Zeiss, who is the man behind the ComfortASP.NET framework see http://www.comfortasp.de/ has published a very interesting comparison on his web site (http://www.daniel-zeiss.de/AJAXComparison/Results.htm) and his blog (http://www.geekswithblogs.com/danielz).
(Have you noticed that there are many German developers involved in AJAX engines and frameworks?)
I have also implemented the simple sample he uses for the comparison at http://www.mathertel.de/AJAXEngine/S02_AJAXCoreSamples/AJAXComparison.htm.
My annotations on these results
The Aspects of AJAX Engine is consuming less bytes on the network that most of the other machines in this comparison.
The size of the transferred bytes is indeed a very interesting aspect. I have not spent any time in reducing this value and I also have no chance to reduce the bytes of a standard SOAP call without leaving this standard and adding the need to implement server specific code.
The initial transfer of the page is smaller than the original ASP.NET version mainly because I do not need any ASP.NET functionality for deploying a working page.
The biggest part of the Engine is the ajax.js file with 21876 bytes because it contains a lot of comments and additional information. I could reduce this page in a view minutes by stripping off all the comments and blank lines down to 12000 bytes and it can be reduced even more. Thats the size that should be used in the comparison because the other frameworks also only send optimized include files.