A: Despite of its age, Common Lisp is still one of the most expressive languages available. It usually comes with powerful interactive environments and makes an ideal tool for both exploratory programming and production use.
A: This depends very much on the problem at hand. Femlisp is slow for several standard problems. On the other hand, it can also perform astonishingly well for other benchmark problems (compared with other unstructured-mesh FEM toolboxes). Furthermore, very often the speed of program development is the decisive factor. Here, Common Lisp cannot be beaten by statically typed languages like C++ or Fortran.
A: Please ask on the femlisp-user mailing list for optimization help.