Not just Allegro, from what I read, Lispworks is also noticeably faster then SBCL (you might be able to find some comparisons for a practical system by googling for elPrep, which is now written in Go but used to be Common Lisp and ran on both Lispworks and SBCL). Concerning the appeal of Clozure CL, I think people have traditionally used it for faster dev turnaround due to better compilation speed than SBCL (which in turn typically generated faster code).