CLOS implementations have had to deal with the problem of extreme dynamism for a long time, so there has been quite a bit of optimization work associated with it. Polymorphic inline caches help a lot, for example.

That said, a good tracing JIT is probably better; but on the other hand, SBCL has profited from many years of optimization work on CMU CL. It would be interesting to see some real-world performance comparisons between SBCL and PyPy.