It might be odd in a chapter on AJAX to say avoid it, but with the seamlessness and power that Ext.NET offers, it might be tempting to code everything in C# and avoid client-side coding. But excessive HTTP requests this causes would affect performance.
However, as an Ext.NET developer has noted, it may be worth trying to minimize the amount of DirectEvents and DirectMethods that are invoked, especially in the cases where the same effect can be achieved purely by JavaScript code, because that will avoid the HTTP roundtrip overhead, thus providing a better user experience.
Of course, many important operations will need to get data and interact with the server, but for other cases, if it can be done on the client side, try to do so. It is therefore worth repeating what was said at the beginning of the book: it is beneficial to invest time learning client-side Ext JS library in addition to Ext.NET to get the best of both worlds and ensure that your Ext...