I am just beginning to look into "AJAX" for an upcoming project.

I have a couple of questions I am hoping someone can help me with.

First, the presentation portion of AJAX is basically building a user interface with javascript. Does anyone know which browser is most optimized for quick execution of js? Most of the sample programs I have seen using AJAX have been rather lathargic compared to similar native Windows applications. I am wondering if a GUI is really possible for an application running in Javascript. I can see it for an occasional use program like checking your e-mail a couple of times a day, but for an 8 hour system interface I am thinking it will just be frustrating for the end user. Is there a particular system speed and memory amount that must be used in order to get good performance out of these applications?

Second, is there any kind of RAD tool that will allow the developer to layout the user interface graphically and that is not tied to a particular back end like JAVA or JSP? I would like to interface with ASP at this point, but I may want to change that in the near future but I don't want to have to trash the interface portion of the system just because I change the back-end.