This thread is wondering why the MP shared_ptr is slower than SP shared_ptr, or in Rust where this distinction isn't compiler magic, why Arc is slower than Rc

So hence the sequentially consistent ordering doesn't come into the picture.

And yeah, no, you don't get the sequentially consistent ordering for free on x86. x86 has the total store order, but firstly that's not quite enough to deliver sequentially consistent semantics in the machine on its own and then also the compiler has barriers during optimisation and those are impacted too. So if you insist on this ordering (which to be clear again you almost never should, the fact it's the default in C++ is IMO a mistake) it does make a difference on x86.