There are a large number of different options that Solr gives you for enhancing performance of a specific Solr instance, and for most of these options, deciding to modify them depends on the specific performance result you are trying to tune for. This section is structured from most generally useful to more specific optimizations.
Solr runs inside a Java Virtual Machine (JVM), an environment that abstracts your Java-based application from the underlying operating system. JVM performance improves with every release, so use the latest version (except the recently released Java 7 that has issues that causes Solr to fail! Wait till a .1 release comes out!)