Beyond that Lem it is quite fast and there is the OpenGL [1] front as unofficial contribution. It could be embedded as a Lisp IDE inside of a game called Lisp Hacking Experience that was never made.
The amount of good reasons for doing that it can be enumerated by thinking about the common lisp ecosystem as I said earlier.
BTW, emacs is great but it not have good defaults enough for modern Common Lisp development, Lem it's awesome as default for Common Lisp ;). Not having any beginner friendly editor for Common Lisp it was one of the reasons that I teaching Lisp was so difficult.
Edit: Portacle is a good Emacs dist, but it is still emacs, so don't push the discussion with this in mind. It's pointless. And Portacle it's great in your own way (https://portacle.github.io/)