Arrays are often the most efficient representation for collections of objects because:
Access of arrays that are not of type simple-array is less efficient, so declarations are appropriate when an array is of a simple type like simple-string or simple-bit-vector. Arrays are almost always simple, but the compiler may not be able to prove simpleness at every use. The only way to get a non-simple array is to use the :displaced-to, :fill-pointer or adjustable arguments to make-array. If you don't use these hairy options, then arrays can always be declared to be simple.
Because of the many specialized array types and the possibility of non-simple arrays, array access is much like generic arithmetic (see section 5.11.4). In order for array accesses to be efficiently compiled, the element type and simpleness of the array must be known at compile time. If there is inadequate information, the compiler is forced to call a generic array access routine. You can detect inefficient array accesses by enabling efficiency notes, see section 5.13.