The idea of Forth is to minimize complexity, so it is more brain work. The idea of Lisp is to handle complexity, so it is more CPU work.
The result of Forth is that one is constantly improving his brain and this leads, on some day, in something like enlightenment. The understanding of why and how solutions can become brutally simple. Without exercising, how should one become better ?
I think Forth and Lisp are very powerful but in very different ways.