For the last year I've been working on a project that has a complex toolchain, such that every time I hit cmd-s I have to wait 2-5 seconds before I can reload the browser to see the changes. It's remarkable how easily I can get distracted in that short interval, especially if I experience it dozens of times per day.

For the last couple days I've been doing Project Euler with Ruby and the lack of lag time translates into much better focus.