[pvg]

Author mentions they didn't use optimization flags but doesn't include the compilation details. You can sort of guess that (relatively) unoptimized C might perform worse than V8's JIT on short, straight computational code - you're more or less testing two native code generators doing a simple thing except one has more optimizations enabled and wins.

[hyperbrainer]

Oh, I assumed he still did -O2, and did not do anything else. Is that bad to assume?

PS: I do not use C beyond reading some of its code for inspiration, so kinda unaware

[pvg]

It just says 'no optimization flags' and that's that, I don't think even the compiler is mentioned so the author is not giving you a lot to go on here - you don't even know what the default is.

A modern C optimizing compiler is going to go absolutely HAM on this sort of code snippet if you tell it to and do next to nothing if you don't - that's, roughly, the explanation for this little discrepancy.