Are both builds threaded?
Yes, I wondered about that, too.
When I ran vr's original code on Ubuntu, with both threaded and unthreaded builds of perl-5.26, the timings didn't alter significantly.
But I didn't have an unthreaded Windows build to test with.
I have now built one (perl-5.26) only to discover that one can't install a usable Inline::C because Win32-IPC (more specifically, Win32-Mutex) doesn't compile.
One can successfully force install Inline::C, but it's unusable.
Unfortunately, Win32::IPC builds using Module::Build - and I simply cannot stomach any troubleshooting that involves Module::Build.
It would have been nice to verify whether an unthreaded build on Windows does access Inline::C functions faster.
Maybe someone else ....