If you are running Liferay with Tomcat, you have most likely come across this annoying message upon startup:

INFO: The Apache Tomcat Native library which allows optimal performance...

This is Apache's subtle hint that you can speed up Tomcat.  I decided to see how much of a boost it would give you.  Without the JNI library installed, I get this on startup of a vanilla 4.3.3, bundled with Tomcat on JDK5:

INFO: Server startup in 14463 ms

after installing the JNI library, I get about a 10% improvement:

INFO: Server startup in 13266 ms

This, of course, is no sophisticated metric.  But, every bit helps.  Try it yourself and let me know what kind of speed difference you are getting.  Also, feel free to update the Wiki I have started on this subject since, of course, I only have the installation working on a Mac.  :)