Daoyue, WebSphere Portal server is a very heavy weight server becuase portal server runs on top of the application server and there are many applications (as part of portal install) are running.
On my machine, I have 2 GB of RAM and I run portal server just fine. The reason it's slow on your machine is most likely that you don't have enough RAM. Here is what you could do: 1. Install portal server as test environment, which tends to be little (not much) lighter. Also, when you install as a test env, you can start server from within RAD. Startup and deployment both tend to be faster this way. Also, most changes in the portlet application don't require a server restart when running in test environment from RAD 2. There are ways to remove unnecessary applications and services. If you have time and energy explore that option 3. Add more RAM
In most cases, you can't run on tomcat becuase tomcat is a servlet container not a portlet container. You may be able to run jetspeed locally and deploy on websphere in production environment BUT you have to make sure that the portlet applications you develop are strictly JSR 168 compliant and hence portable between various portlet containers. This isn't an easy task and will likely take more time as things may not work in production as expected. Not to mention it's also a risk as your dev environment will be significantly different from the prod environment.
Hope this helps. Let me know if you have more questions.