performance

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.

Increase realpath_cache_size to get rid of many many needless lstat calls.

Based off of Javascript Aggregator this module will gzip your aggregated css files. For Drupal 6.x
Advantages

• It's compatible with Apache 1.3 (works great with cheap hosting).
• Compresses the content once and saves the result (less CPU load on server).
• Uses level 9 compression because it's only run once per file (smaller file size).
• In general, all you need to do is enable the module and it works (no core hacking/modifying .htaccess).

Disadvantages