CLOS is currently slower, but there are some cool ideas floating around to greatly speed it up. The roadblock has been CLOS's dynamism: everything needs to be redefinable at run time, even the classes. But it's possible for an implementation to get high efficiency even with that, using separate code snippets that are reached by a single branch (although I don't know if any CL implementation does this yet):

http://metamodular.com/SICL/call-site-optimization.pdf

(I think a modern version of Common Lisp would take this idea and run with it, making most or all built-in functions generic.)

CLOS really shines when it comes to problems that benefit from method combination, which allows one to decompose computations into reusable nuggets that can be reassembled using multiple inheritance.