Optimising SQL queries Oskar P
This article includes quite a lot of detail about how to optimise SQL queries, covering indexes, index cardinality, index hinting, and more. In my experience, most of the time a site has performance problems it seems to be a poorly-indexed database table, or a query that can never use an index. Have a read of this to figure out what you can tune to resolve these sorts of issues.
Fast Software, The Best Software Oskar P
A compelling discussion on how software speed can be a proxy for overall software quality and focus.