Premature optimization is the root of all evil. -- D.E. Knuth
Efficient code can be well-structured and clean code, based on on a sound overall architecture and sound algorithms. Efficient code can be highly implementation-code that bypasses documented interfaces and takes advantage of obscure quirks in the current implementation.
Ideally, your code should only contain the first kind of efficient code. If that turns out to be too slow, you should profile the application to find out where the performance bottlenecks are and optimize only the bottlenecks. Other code should stay as clean as possible.
Fortunately, compiler and run-time optimizations introduced in R12B makes it easier to write code that is both clean and efficient. For instance, the ugly workarounds needed in R11B and earlier releases to get the most speed out of binary pattern matching are no longer necessary. In fact, the ugly code is slower than the clean code (because the clean code has become faster, not because the uglier code has become slower).
This Efficiency Guide cannot really learn you how to write efficient code. It can give you a few pointers about what to avoid and what to use, and some understanding of how certain language features are implemented. We have generally not included general tips about optimization that will work in any language, such as moving common calculations out of loops.
It is assumed that the reader is familiar with the Erlang programming language and concepts of OTP.