There are several things to consider in improving the performance of Liferay:

1. Properties
There are several configurations on-ext.properties portal that can improve performance:
last.modified.check=false
Disable checking last modified date on server side CSS and Javascript
theme.css.fast.load=true
Load the theme’s merged CSS files for faster loading for production
javascript.fast.load=true
Load the javascript files for faster loading for production

2. Servlet Filters
Disable all unused servlet filter
 SSO CAS
 SSO NTLM
 SSO OpenSSO
 Sharepoint Filter
Comment all servlet filters are not used in the file: liferay-portal-6.0.6\tomcat-6.0.29\webapps\ROOT\WEB-INF\web.xml

3. Portlet
Disable all the portlets that are used in a way comment on the file: liferay-portal-6.0.6\tomcat-6.0.29\webapps\ROOT\WEB-INF\liferay-portlet.xml

4. Memori
Increase JVM Tuning by editing the file: \liferay-portal-6.0.6\tomcat-6.0.29\bin\setenv.bat

JAVA_OPTS="$JAVA_OPTS -Dfile.encoding=UTF8 -Duser.timezone=GMT -XX:NewSize=700m -XX:MaxNewSize=700m 
-Xms2048 -Xmx2048m -XX:MaxPermSize=256m -XX:+UseParNewGC -XX:+UseConcMarkSweepGC 
-XX:+CMSParallelRemarkEnabled -XX:SurvivorRatio=20 -XX:ParallelGCThreads=8"

If memory servers over 8 GB with 4 processors.

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