You haven't given us enough information to give you performance
advice.

If you anticipate a lot of client-side calculations that can't be done
in javascript, then you probably want a Java or Applet. Consider Java
WebStart for this purpose.

On the other hand, if you can keep all the heavy work done on the
server side, and you want a web-based application, use Servlets (Not
just JSP, look into frameworks, such as Spring, with Spring Web Flow,
or Struts, etc...)

Also, if you have to ask "Which technology should I use?", you are NOT
ready to ask "What do I need to use for performance?", If you have a
clear understanding of your business requirements, then the underlying
technology should be almost obvious.

Good Luck,
Daniel.

I think it is unlikely that there will be a noticeable
difference in performance.

The two most important drivers for the decision is:
  - functionality, a client app can provider a much richer
    user interface than a web app
  - distribution, everybody has a browser so a web app
    does not require any install, a client app needs
    to have Java installed on the system and the app
    itself

Arne