There are a lot of articles explaining the basics in performance optimization in ASP.NET. Here is one and here is another. It is important to know the basics and use them where appropriate. The basics include caching, viewstate optimization etc. But what do you do to optimize further, when you have implemented all the basic techniques? You search the Internet to find an article about it, just like this one. The conclusion of this article is: Make the response from the server to the client as small as possible. You already have made the server-side code run fast with the basic techniques, so now you need to shrink the output. Here are a few examples that will take you a long way.
1. Remove whitespace in the HTML
This is very simple to do in ASP.NET. Here is the method to use.
It can easily reduce your html code by 15%.
This is also very simple. Read this post on how that is done easily.
3. Put all styling in a stylesheet file
If you do a lot of styling in your html, the file size increases. Every time a page loads the styling is also returned from the server. But if you put all your style in a stylesheet, it will get cached by the browser and thereby only return from the server once. This can be a great improvement if you have a lot on in-code styling.
If you apply all four techniques, my guess is that you have reduced the response to a minimum. If you pay for the bandwith used by the website, this could save you a lot of money by reducing the filesize.