It's always easy to write 1% slower Ruby code without knowing it. 1% slower code can be a big problem when there are hundreds of them, and it's not easy to find and fix all of them afterwards.

Benchmarking and profiling every single change ("Benchmark-Driven Development") helps out here! This idea may sound absurd and inefficient, but I have written a 100x faster Sinatra implementation in this fashion. I will introduce recent evolution in profilers which support this style.

Your programs will be faster, even before you can say premature optimization is the root of all evil!