The reason is that Clasp doesn't do Lisp language level optimizations like escape analysis yet. All bindings are stored on the heap and the stack/registers are underutilized. LLVM is a great library for implementing C and C++ but more work needs to be done to support Lisp features like closures and first-class functions. We are working on that now. The goal was first to "make it correct" and now we will "make it fast". Once we have a faster compiler (give us a couple of months) I don't see why it couldn't approach the speed of SBCL (a tall order).