Julia's JIT is a just ahead of time method jit just like SBCL, and the language is designed around making idiomatic code as fast as possible with that approach.
So every function is a multimethod that gets compiled the first time any combination of types is first encountered, and at that point the type information is propagated to all function calls inside the method body, to eliminate dynamic dispatch on those & possibly inline the appropriate method. Julia has parametric polymorphism as well, so it's common for all types in the body to be inferable.
As a result, Julia code tends to make much more heavy use of polymorphism & multimethods than common lisp (where CLOS has a significant runtime cost instead of being a near-zero cost abstraction), since the language and the runtime were designed carefully together to make that fast.
So every function is a multimethod that gets compiled the first time any combination of types is first encountered, and at that point the type information is propagated to all function calls inside the method body, to eliminate dynamic dispatch on those & possibly inline the appropriate method. Julia has parametric polymorphism as well, so it's common for all types in the body to be inferable.
As a result, Julia code tends to make much more heavy use of polymorphism & multimethods than common lisp (where CLOS has a significant runtime cost instead of being a near-zero cost abstraction), since the language and the runtime were designed carefully together to make that fast.