Custom JSP taglib lets you implement some intermediate pages for your long time calculated JSP code (some like "Wait please ..."). So if the response from your JSP page takes long time to return, you can return an intermediate page with say an animated gif, some text information etc. When the results become available this intermediate page will be automatically replaced with the original output. For example, suppose you have some very long time calculated JSP code heavy_code.jsp. You can do so:
 

Try It Out: http://www.servletsuite.com/servlets/waittag.htm