Better Response Times on WebSphere Cluster

Hi,
We have our web application deployed on a Cluster of 2 IBM WebSphere 4.0.4 Application Servers on
Sun Solaris boxes. Clustering was achieved using "Horizontal Cloning" of the Application Servers.
Our application does not have EJBs and the objects in session are persisted on a DataBase. The
problem we are facing is POOR RESPONSE TIMES. We have tuned the
1. Connection Pooling parameters for both Application and Session DataSources.
2. Web Container threads on both the machines.
When Load/Performance Testing was done using Rational Robot, the response times were not
satisfactory as compared when the application was deployed on a Single Server. This defeats the
very purpose of going for a Custer - Scalability with minimal Response Times.
Any help/pointers/suggestions with respect to WebSphere Cluster will be greatly appreciated.
Thanks and Regards,
Sudharshan Govindan