Ajax won't slow down your page. In fact, it will make it appear to load more quickly because it can allow you to draw portions of the screen when they are ready instead of waiting for the entire screen to be done before rendering.
You do need to be careful to differentiate what type of Ajax you are using. ASP.NET has an Ajax version and so does jQuery. In the newer versions of ASP.NET, it uses the jQuery Ajax.
Here is more information related to your question:
http://forums.asp.net/t/1390408.aspx[
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