I just noticed near the bottom of your post, where you said:
Program makes a lot of calculations with packed decimal.
There is a big difference in the way ILE C implements support for "packed decimal" as a "native" data type -- versus the way it is "supported" in ILE C++ -- in ILE C, the compiler directly genrerates in-line code (in WCode / NMI) to work on packed decimal data. In the ILE C++ compiler, support for "packed decimal" is through a C++ library called "BCD" -- there are a lot of macros and such defined in the QSYSINC/H include member named BCD ... if you take a peek at that member, I think you will see that this uses "operator overloading" in C++ to trick the compiler into calling the desired library functions when operations are performed on "packed decimal" data.
This difference could easily account for the big differences you are seeing.
And then, you said:
... is there anything that can be done about it?
In C++, you could look into converting the packed decimal data into 64-bit integers, and then performing any calculations using those long integers. This would be much faster than using packed decimal arithmetic, then convert back to packed decimal format, if needed. Or, just stick with ILE C/400 for those programs that require doing a lot of packed decimal arithmetic operations.
Hope that helps,
Mark S. Waterbury