One of the major changes in Drupal 6 (where "major" is defined as "worthy of a mention in Dries' keynote") was a new feature of the menu and theme hooks. The newly introduced "file" and "file path" keys in those hooks' respective retun arrays. allow them to define files that get included conditionally, only when needed. In theory, that should be a big performance boost; page handlers are virtually never called except for on the page they handle, so loading all of that code on every other page is a waste of CPU cycles. Of course, there is also the added cost of the extra disk hit to load that one extra file we need. Modern operating systems should do a pretty good job of caching the file load, but that may vary with the configuration.
So just how much benefit did we get from two dozen fragile patches that were a glorified cut and paste? And is it worth doing more of it? Let's benchmark it and find out.
The day is nearly upon us! Drupal 7 will open up developers to PHP 5 functionality when it is released next year. Already, there is talk of how, and if, to leverage PHP 5's object handling now that we don't need to deal with the weirdness of PHP 4's object model. Of course, because it's Drupal, our army of performance czars want to know just what the cost is for object handling, and especially advanced object magic like __get(), __call(), the ArrayAccess interface, and so forth.
So let's find out. :-)