It always amazed me how fast “yes” is. Whenever I need to generate a really big file for whatever reason (e.g. testing multipart file uploads), I will run something like “yes > myfile.txt” for a few seconds.

Similarly, I tried writing a naive version in node.JS a few years ago and it was several orders of magnitude slower than coreutils…, which is usually the case whenever I try and clone one of the “simple” utilities from coreutils.