This makes sense. It's like when I'm solving a project euler problem, I don't use verbose output because it runs much faster without having to print everything to the screen. (Python)
That's exactly my experience. Once I wrote some naive PE solution in nodejs and added some logging at each step. After few minutes and far from a solution I stopped it and disabled logging. I got the solution within seconds. The diff was few orders of magnitude, which was pretty surprising for me.
Sometimes writing to the terminal can be the slowest part because it's unbuffered by default in an interactive prompt, so even just redirecting stdout to /dev/null (or even a file) can be faster. Had that happen some ten years ago, so my explanation could be out of date or misremembered but the effect was real.
Your explanation is correct. That is the semantics of Unix stdio buffering as implemented by glibc. So across a wide variety of languages, you'll see a significant performance difference if you are doing lots of writes to the terminal.
On the other hand if you didn't do this, then interactive terminal programs would be entirely unusable.
