k is really fast. Half of the things on the Shakti mailing list are just Arthur getting really excited about how significantly he's beating x or y or z in performance and giving numbers for it. `grep`ping it now I see 40 in half a year that explicitly contain the word "benchmark," though not all of these are comparing to other things (some are just comparing to different k releases), and there are more comparisons without that word.
Arthur doesn't work at Kx anymore, by the way. He's at Shakti now. Shakti has a different (but still draconian/non-(A)GPL) license. It probably doesn't have the benchmark clause, but I don't care enough to check (I prefer J to k and don't have a proprietary k on my system).