In CMU Common Lisp, as is any language on any computer, the path to efficient code starts with good algorithms and sensible programming techniques, but to avoid inefficiency pitfalls, you need to know some of this implementation's quirks and features. This chapter is mostly a fairly long and detailed overview of what optimizations Python does. Although there are the usual negative suggestions of inefficient features to avoid, the main emphasis is on describing the things that programmers can count on being efficient.
The optimizations described here can have the effect of speeding up existing programs written in conventional styles, but the potential for new programming styles that are clearer and less error-prone is at least as significant. For this reason, several sections end with a discussion of the implications of these optimizations for programming style.