EXWM is ;). I used it for about a year, but ultimately found the performance, and in particular a low language server locking up the window manager, unacceptable.
I just switched to exwm and I'm curious if the article will teach me some tricks which might be useful there. My initial experience is good but it is still a bit rough around the edges. In other words, I have not yet found my best configuration and workflows.
StumpWM uses Common Lisp, and can be setup to be nearly identical to Emacs window management. It is also quite performant. Out of the box, its configuration is pretty nice already.
