Why deallocating heap memory is much slower than allocating it?

This is an empirical assumption (that allocating is faster then de-allocating).

This is also one of the reason, i guess, why heap based storages (like STL containers or else) choose to not return currently unused memory to the system (that is why shrink-to-fit idiom was born).

And we shouldn't confuse, of course, 'heap' memory with the 'heap'-like data structures.

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So why de-allocation is slower?

Is it Windows-specific (i see it on Win 8.1) or OS independent?

Is there some C++ specific memory manager automatically involved on using 'new' / 'delete' or the whole mem. management is completely relies on the OS? (i know C++11 introduced some garbage-collection support, which i never used really, better relying on the old stack and static duration or self managed containers and RAII).

Also, in the code of the FOLLY string i saw using old C heap allocation / deallocation, is it faster then C++ 'new' / 'delete'?

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P. S. please note that the question is not about virtual memory mechanics, i understand that user-space programs didn't use real mem. addresation.