What are all the important areas where performance of an application can be increased?

application level (refactoring ,enhancing code)
database level (optimizing queries)
operating system level (to monitor and alter harware resources)
network level (to enhance or effectively utilize)

1.www.codecollege.NET 2.http://www.interviewsguru.info 3.The Encylopedia of Web Sites 4.Blogging and Earning