That basically means Lisp is probably about as fast as C because: C that is portable, easy to maintain, and robust against bad inputs also only runs at 50% of the speed of C.
If you take any fast C program and add all the logic that is missing to make it robust and secure, and refactor it so that it is halfway nimble against changing requirements, it's no longer running at the mythical speed of C.
If you take any fast C program and add all the logic that is missing to make it robust and secure, and refactor it so that it is halfway nimble against changing requirements, it's no longer running at the mythical speed of C.