An important caching technology to ensure good performance in WebSphere Portal is fragment caching. Portlet html fragments are cached for reuse for subsequent requests.
Due to an issue with the install scripting in Portal 7 and 8 fragment caching on any secondary nodes in a cluster was not working. This issue was fixed with CF15 for WebSphere Portal 184.108.40.206 and 220.127.116.11 and CF1 for Portal 8. If you created the secondary node(s) though before having the fixes applied fragment caching on secondary node(s) will not work.
To verify if you are having the issue - check if the file cachespec.xml exists in the <profile>/properties directory on the secondary node(s). If the issue exists on your system and you are using fragment caching you should copy the file from the primary node and restart to fix the problem. To verify that the issue is solved you can leverage the WebSphere Application Server Extended Cache Monitor. After installing it, check the base cache to ensure that fragment cache entries are inside the cache after hitting a page that has portlets that use fragment caching. A sample entry could look like this:
/wps/PA_WCMLRingPortJSR286/Web Content Viewer (JSR 286):Z7_18GAHIS0I8T970IBKLE1L20NR4:anonymous:text/html:en, en-US;q=0.9:ZM_CGAH47L0004820IDBHD79M0047:UTF-8:normal:PUBLIC_CONTEXT=mylibrary/content/welcome&pcid=032scfc-42c5-4342-8502-bfaa9c365f82:UTF-8:requestType=GET
You can find more information about the WebSphere Application Server Extended Cache Monitor here: http://www.ibm.com/developerworks/websphere/downloads/cache_monitor.html