Vaadin Perormance

Hello all vaadin developers,
I developed an application in vaadin 6. the application is using by multiple user at a time (almost 2 to 3). when a user is loged in, he is continusly doing database transations through application.
some time the speed of application becomes very slow. please recommed me what should I do. also recommed me Ram size and heap size for a multi user vaadin application (as I am using Tomcat Server). and how can i speed up my application.