Why calling via weak_ptr is so slow?

I have read the question What's the performance penalty of weak_ptr? but my own tests shows different result. I try to make delegates with smart pointers and now I am testing it. Simple code shows the great perfomance issue with weak_ptr. Does anybody could say why?

#include <chrono>
#include <functional>
#include <iostream>
#include <memory>
#include <stdint.h>
#include <string>
#include <utility>


struct Foo
{
    Foo() : counter(0) { incrStep = 1;}

    void bar()
    {
        counter += incrStep;
    }

    virtual ~Foo()
    {
        std::cout << "End " << counter << std::endl;
    }
private:
    uint64_t counter;
    uint64_t incrStep;
};


void pf(const std::string &md, const std::function<void()> &g)
{
    const auto st = std::chrono::high_resolution_clock::now();

    g();

    const auto ft = std::chrono::high_resolution_clock::now();

    const auto del = std::chrono::duration_cast<std::chrono::milliseconds>(ft - st);
    std::cout << md << " \t: \t" << del.count() << std::endl;
}


int main(int , char** )
{
    volatile size_t l = 1000000000ULL;
    size_t maxCounter = l;

    auto a = std::make_shared<Foo>();
    std::weak_ptr<Foo> wp = a;

    pf("call via raw ptr        ", [=](){
        for (size_t i = 0; i < maxCounter; ++i)
        {
            auto p = a.get();
            if (p)
            {
                p->bar();
            }
        }
    });

    pf("call via shared_ptr      ", [=](){
        for (size_t i = 0; i < maxCounter; ++i)
        {
            if (a)
            {
                a->bar();
            }
        }
    });

    pf("call via weak_ptr       ", [=](){
        std::shared_ptr<Foo> p;
        for (size_t i = 0; i < maxCounter; ++i)
        {
            p = wp.lock();
            if (p)
            {
                p->bar();
            }
        }
    });

    pf("call via shared_ptr copy", [=](){
        volatile std::shared_ptr<Foo> p1 = a;
        std::shared_ptr<Foo> p;
        for (size_t i = 0; i < maxCounter; ++i)
        {
            p = const_cast<std::shared_ptr<Foo>& >(p1);
            if (p)
            {
                p->bar();
            }
        }
    });

    pf("call via mem_fn         ", [=](){
        auto fff = std::mem_fn(&Foo::bar);
        for (size_t i = 0; i < maxCounter; ++i)
        {
            fff(a.get());
        }
    });

    return 0;
}

Results:

$ ./test
call via raw ptr            :   369
call via shared_ptr         :   302
call via weak_ptr           :   22663
call via shared_ptr copy    :   2171
call via mem_fn             :   2124
End 5000000000

As you can see, weak_ptr 10 times slower than shared_ptr with copying and std::mem_fn and 60 times slower than call by raw ptr or shared_ptr.get()