FWIW, i'll submit a recent example of SBCL, applicable for Lisp in general i think, too): a friend of mine wanted to do some linear algebra in real time. The scenario was that he has some sensors recording human movement live and would like to compute some quantities on the fly. He used sympy to precompute the operations to be carried out symbolically, using the "export to C++" function of sympy. Both gcc and clang choked on the large file produced (unless he turned off optimizations completely). Using emacs, I transformed the file into sexps, which were then parsed and compiled into separate lambdas (a process which took about 20 secs -- gcc/clang were taking about 5 minutes before giving up). With only a few type declarations, the lisp version of the update was about twice faster than the C++ counterpart, but I left it there, since due to external (management) constraints, Lisp would not be acceptable.