I heard that maintaining everthing in a sesson object will reduce the performance of the server. Is this true?.
Short answer: NO
Longer answer - it depends - this is another of the many preposterous generalizations that contaminate the servlet/jsp programming world.
The purpose of a session is to hold on to the unique data for a particular user's interactions with a web application. The servlet API provides for other mechanisms for holding resources specific to individual servlets, to web applications, and to entire servers.
Any more discussion of the use of sessions would require more information about your particular application.
Bill