Below is a list of addons that help workaround known performance potholes in plone, many of which have been fixed in the latest version. A lot of us are stuck on our good old plone installs so I'm keeping this page around for you. And me.
Be aware that these packages can be for experienced peeps only, since they require a good understanding of the problem in order to configure the fix.
experimental.opaquespeedup helps with unnecessary event overhead
collective.indexing takes a lot of the tools that you see above for plone 2.5 and combines them for plone 3
collective.solr - if you are not happy with the speed of indexes, you can install another index to take over your SearchableText indexes called solr (Apache/Lucene). While it won't take over the management of solr for you, it will integrate it filly into Plone.
unimr.compositeindex - "CompositeIndex is a plugin index for the ZCatalog. Indexes containing more than one attribute to index an object are called "composite index". Such indexes should be created if you expect to run queries that will have multiple attributes in the search phrase and all attributes combined will give significantly less hits than the any of the attributes alone. The key of a composite index is called "composite key" and is composed of two or more attributes of an object."
Chameleon is supposed to make page template rendering much faster by compiling ZPT down to bite code. I haven't used this so I'm not sure on version requirements/actual benefits.