[akjj]

I think others have addressed the differences between Python's class system and CLOS, both of which are very flexible, as I understand.

However, as far as optimization, the point is that Python's class system is implicated in almost every line of code. Most operators are actually invocations of corresponding "dunder" methods on their operands, which can be potentially changed, and whose invocation is actually surprisingly complicated and difficult to optimize.

Common Lisp, of course, does not have operators in the same sense, just functions. However, the analogous functions, like +, *, aref, etc. are not generic functions in the sense of CLOS. They only take built-in data types as arguments and cannot be overloaded. This lack of extensibility makes it easier for the compiler to know what actual code is being invoked and to optimize their use. Arguably, this makes Common Lisp seem like a less flexible language, and in some ways its design does pay more attention to optimization than its reputation would have you believe. On the other hand, the lisp syntax means that there's no such thing as a finite set of operators that you'd want to overload. If you want a different type of multiplication, you can just use a different function.