Comment by oivey

I'm not sure it's common knowledge, but it is general knowledge. Not all HNers are writing web apps. Many may be writing truly compute bound applications.

In my experience writing computer vision software, people really struggle with the common sense of how fast computers really are. Some knowledge like how many nanoseconds an add takes can be very illuminating to understand whether their algorithm's runtime makes any sense. That may push loose the bit of common sense that their algorithm is somehow wrong. Often I see people fail to put bounds on their expectations. Numbers like these help set those bounds.