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I would be very very surprised if printing "2971215073" accounted for more than 1 second of runtime.

Well, I know that printing stuff to console can be ridiculously slow, so this doesn't surprise me much.

An entire second to print 10 characters?! How does a terminal that slow even exist?

std::endl flushes which isn't cheap, you're generally better off simply steaming in a newline.

Most of it won't be because the C++ constexpr version has the same IO call and runs in <0.1s.

I think that's because even the std::cout call could be precalculated, so no runtime string concatenation and stream magic is involved.