>>guenth+(OP)
Steel Bank Common Lisp is the most performant (by speed of compiled code) and stable open source implementation, and arguably the most standard compliant and bug free of any Common Lisp implementation, free or otherwise. It also is available free of charge.
I mostly wanted to know of any technical obstacles so SBCL could be improved. If I had to wildly guess, maybe GC performance? SBCL was behind ACL on that many years ago (on both speed and physical memory requirements) the last time I made a comparison.