There's talk about using off-the-heap data structures in one of the most successfull software piece written in Lisp, AllegroGraph.
"Escape from the Heap: Low-Level Programming in Common Lisp" https://www.youtube.com/watch?v=S7nEZ3TuFpA
"Escape from the Heap: Low-Level Programming in Common Lisp" https://www.youtube.com/watch?v=S7nEZ3TuFpA