If you are referring to fastest in terms of wall & CPU time running. Without spending a lot of time optimizing your code yourself, I suspect SBCL would take the crown for vast majority of cases. If you do hyper optimize, the gap with scheme largely closes (sometime scheme wins, sometimes SBCL). So I'd go with SBCL if all else is the same feature wise to you. That said, the last serious benchmark I did of lisps was ~2010 so optimization may have changed since then and of course they were not very rigorus benchmarks (just my compute-bound workload at the time, that primarily boiled down to mostly integer ops). Debian language benchmarks game has some benchmarks written for sbcl and racket that may be good for comparing implementations yourself if interested. https://benchmarksgame-team.pages.debian.net/benchmarksgame/...

SBCL, probably. Scheme would be a close second depending on the implementation (Chicken, Gerbil or other compile-to-C impls.) Racket and Clojure are closer in performance to scripting languages like Python.