PACKED-DECIMAL (COMP-3)
In IBM® Enterprise COBOL Version 4 Release 2 Performance Tuning, it says: "When using PACKED-DECIMAL (COMP-3) data items in computations, use 15 or fewer digits in the PICTURE specification to avoid the use of library routines for multiplication and division".
Using
V6 and the options ARCH(8 | 9 | 10 | 11) and OPT(1 | 2), the compiler
can generate inline decimal floating-point (DFP) code for some of
these larger multiplication and division operations. The maximum intermediate
result size supported for this optimization is 34 digits. Although
there is some overhead in this conversion to DFP, it is less of a
penalty than having to invoke a library routine. This is also true
for external decimal (DISPLAY and NATIONAL) types that are converted
by the compiler to packed decimal for COMPUTE statements.
Using V6.2 and ARCH(12), the compiler instead uses the
vector packed-decimal facility, which accelerates packed and zoned
decimal computation by storing intermediate results in vector registers
instead of in memory. This avoids the overhead of conversion to DFP.