<iostream>: Bad performance of wcout operator<< compared to wprintf

If I compile the following code:

for(auto idx = 0; idx < 10; idx++){
        const wchar_t* a = L"this is a test\n";      
        std::wcout << a;
}

it needs round about 18 milliseconds during runtime.

But if I use C functions like:

for(auto idx = 0; idx < 10; idx++){
        const wchar_t* a = L"this is a test\n";      
        wprintf(L"%s", a);
}

It needs only 5 milliseconds!
Why is std::cout / wcout so slow? I would assume that the C function should be slower because it has to analyze the format string, but...

I tried to optimize it by calling first: std::ios::sync_with_stdio(false);
But this seems to be completely ignored (I found no reference to _Sync...)?

Start by reproducing the two provided wcout and wprintf loops, including the std::ios::sync_with_stdio(false) call. Trace the iostream/wcout implementation and the sync_with_stdio path to identify whether the reported difference is reproducible and what behavior, if any, should be changed; done means a confirmed diagnosis with a focused regression test or a documented reason no code change is needed.