We went through the main js file today and implemented two simple techniques to (hopefully) improve performance.  We traded our javascript concatenation for the Atlas Sys.StringBuilder, and we 'cached' some of our objects.  Here's an example that incorporates both techniques:

We changed something like this:

for (var i=0; i < someArray.length; i++)
{
   var s = someArray[i].Name + someArray[i].Rating;
}

to:

for (var i=0; i < someArray.length; i++)
{
   var s = new Sys.StringBuilder();
   var movie = someArray[i];
   s.append(movie.Name);
   s.append(movie.Rating);
}