SBCL is not as "high-level", especially when you are aiming for speed. Yossi already wrote about this http://www.yosefk.com/blog/the-high-level-cpu-challenge.html , so there is no need for me to do it:

    If you use Lisp’s static annotation 
    system, your code becomes uglier than 
    Java, and much less safe (I don’t think 
    Lisp does static checking of parameter 
    types, it just goes ahead and passes you 
    an object and lets you think it’s an integer).

Additionally, Lua only has "one data structure type" - that's also "higher-level" than what you have to do in Lisp, deciding among singly-linked lists, arrays, hash tables, binary trees, lexical trees, etc.

So while SBCL has its place, there's something about Lua you have to admire, don't you think?