This example is a mistake that is easy to make. The programmer here did not declare his arrays as fully as he could have. Therefore, each array access was about as slow as a function call when it should have been a few instructions. The original declaration was as follows:
(proclaim '(type (array fixnum *) *ar1* *ar2* *ar3*))
The three arrays happen to be of fixed size, which is reflected in the following correct declaration:
(proclaim '(type (simple-array fixnum (4)) *ar1*)) (proclaim '(type (simple-array fixnum (4 4)) *ar2*)) (proclaim '(type (simple-array fixnum (4 4 4)) *ar3*))
Altering the faulty declaration improved the performance of the entire system by 20%.