Which Lisp? What applications do you think it is slow at? Hint: It can be faster than C due to compiler macros, and infact, its regular expression engine is much faster than Perl's, which is written in C, just to give a concrete example.
My experience is that even writing "idiomatic", CLOS heavy code tends to be faster out of the box than most dynlangs out there. Writing close to C++ code requires a lot of manual work and is probably not even worth it unless for optimizing hot-paths.