profiling

script to figure out which functions/methods increase the memory usage a lot.

The next thing to do is to identify the actual bottlenecks. yslow and firebug can help you for front end performance - which are often a bigger issue in the wild, but while Drupal core can make those problems easier to solve (reverse cache support, CDN support etc.), they need real world situations to be measured in, and the actual solutions are often site specific. So here I'm going to focus on what you can find on your local development environment - and three tools, devel module, ab, and xdebug/cachegrind.

Webgrind is an Xdebug profiling web frontend in PHP5. It implements a subset of the features of kcachegrind and installs in seconds and works on all platforms. For quick'n'dirty optimizations it does the job.