At the code level, there is an awful lot you can do to make your scripts execute as quickly as possible. No matter how many GHz you have powering your server or how well-tuned your installation is, if you have sloppy code your application will crawl.
Despite the speed of databases, any queries you perform are likely to be the slowest part of your script unless you make special effort to optimise your data. Use indexes wherever applicable, use smart queries that do not chew up resources pointlessly, and take full advantage of the various memory buffers available to you.
Introducing caching solutions, whether that be soft caching (using a product such as Zend Performance Suite) or hard caching (saving generated files as HTML), will make a big speed difference and help you keep costs down. Remember, though, no one wants to share a shopping basket with someone else - only use hard caching if you can get away with it!