As other's have said, K (the language and the interpreter) is highly optimized for processing tabular data. I've heard of it outperforming C in processing billions of rows of financial data. (Which, by my understanding, is K's primary market.)
Lastly, K (like APL, J, A+, Klong, Kona, etc.) is very terse. A phrase I've seen online is "one line of K is roughly equivalent to 100 lines of C". K programs, like APL & co., very often fit entirely into a screenful of text, but require close and complete reading to understand. There are lots of videos on youtube of people writing literate sudoku solvers or conway's game of life in a dozen lines or so, and terse solutions in a dozen or so characters.
People who like these programming languages tend to like their terseness -- they can view the entire program all at once (no scrolling or searching), and refactor or rewrite the program in a few seconds of minutes of typing.
The article says you can do the same with cpp and some extra libraries, but I wouldn't immediately think cpp or java was easily beatable -- manjana wonders --