Hacker News Reader

Most likely.

http://www.spreeder.com/ says: "To train to read faster, you must first find your base rate. Your base rate is the speed that you can read a passage of text with full comprehension. We've defaulted to 300 wpm, showing one word at a time, which is about the average that works best for our users. Now, read that passage using spreeder at that base rate."

I wouldn't be surprised if Medium is calculating read time the same way. It's just aesthetically pleasing and somewhat useful.

If I was doing this I'd also scale by a readability score, like the Gunning Fog index:

http://en.wikipedia.org/wiki/Gunning-Fog_Index

http://www.readability-score.com/ is an online calculator that will check your text and give you an average of a few scales like the Gunning.

This is what I assumed just wondered how they decided what the average words per minute is.