At one point, I was planning for this document to become a book on PostgreSQL performance tuning. Other projects having come up, I'm releasing what's here as it is.
You may wish to start with the " QuickStart"; it represents a set of things you can do to improve performance that do not involve coding changes. An hour or two spent improving database configuration, and VACUUMing and ANALYZEing some tables may cheaply provide substantial performance increases without any need to get into application changes.