5.11.9 Specialized Structure Slots

Structure slots declared by the :type defstruct slot option to have certain known numeric types are also given non-descriptor representations. These types (and subtypes of these types) are supported:

(unsigned-byte 32)
single-float
double-float
(complex single-float)
(complex double-float)

The primary advantage of specialized slot representations is a large reduction spurious memory allocation and access overhead of programs that intensively use these types.