Premature optimization is the root of all evil (or at least most of it) in programming.
—Donald Knuth (attributed to C. A. R. Hoare)
Performance is an interesting beast. Performance optimization often has a bad reputation because it is often performed too early and too often, usually at the expense of readability, maintainability, and even correctness. Rails is generally fast enough, but it is possible to make it slow if you are not careful.
You should keep the following guidelines in mind when optimizing performance:
It is very tempting to try to squeeze every last bit of speed out of a piece of code, but often you can miss the bigger picture. No amount of C or assembly tweaking will make bubblesort faster than quicksort. Start your optimization from the top down.
Your code should be first easy to read and understand, and only then optimized for speed.
Typically, the code profile has a lopsided distribution: 80% of the time is spent in 20% of the code (for some value of 80% and 20%). It makes sense to spend your limited resources optimizing the sections that will bring the greatest gain in performance.
The only way to be certain about where your code is spending its time is to measure it. And just as in carpentry, you can waste a lot of time if you make changes without being very sure exactly what those changes should ...