Personally, I find algorithmic improvements much more interesting than caching-based solutions.
In addition to "algorithmic improvements" there's also the "programmatic improvements". I've posted a few comments on this subject but they, so far, have garnered zero response, possibly because they talk about things with are "out of scope" for (imho) 99% of Drupal sites anyway. An simple example is the MySQL INT type. The MySQL C API returns everything as a string regardless of how the DB actually stores the data. So, you can be more prudent in how you use these types in PHP userspace rather than doing type conversations (the ctype_*/is_numeric issue recently an example of this). PHP is a great language for "getting things done" but the convenience can come as a performance price. These sort of issues only pertain to a small minority of cases but if those few cases are often called they tend to add up in the long run. best regards, --AjK