[proffeatur8]

And then there’s the little issue of over a magnitude improvement in performance in Julia compared to the already-optimized Common Lisp code…

[forgotpwd16]

True. Julia makes it easier to write performant code. Plus there're nice facilities enabling further optimization.

[reikonomusha]

On average, you will not get anything close to order of magnitude speed-ups on optimized Lisp code.

[eigenspace]

This really depends. If someone is trying to operate with small user defined generic functions on a large array of user defined defined classes in Common Lisp in a tight loop (i.e. what Michael was doing) then performance is very very hard to recover.

Of course, performance claims about Common Lisp are very slippery because one can always say "well, __ implementation using ___ library can avoid ____ specific performance problem", but I think it's fair to say that in general, generic code and CLOS features can land you in very serious performance trouble quite quickly, especially if you're trying to make reasonably portable code.