Exceptionally fast code can be written in any language. The question is where the “line of convenience is” drawn. Common Lisp (and other Lisps in the past) have made it convenient to stop coding when the function performed its job correctly. C and C++ have made it convenient to stop coding when the function performed its job quickly. Unless you are willing to continue coding past the “line of convenience” to quick Common Lisp and correct C/C++ code, you are comparing apple-tree flowers and rotting oranges for edibility.