Const lvalue reference vs forwarding reference

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C++14

I just cannot understand why would one want to use a forwarding reference instead of a const lvalue reference.

So why instead of this:

template<typename T>
    void g(T&& x)
    {
        ...
    }
    
    template<typename T>
    void f(T&& x)
    {
        g(std::forward<T>(x));
    }

I could not use the following and get the same results performance wise (assuming I dont want to modify object x):

template<typename T>
    void g(const T& x)
    {
        ...
    }
    
    template<typename T>
    void f(const T& x)
    {
        g(x);
    }

What I have tried:

This is a theoretical question