A more efficient way than function reference?

I have a class that uses a reference to a function:

double u( const double& x, const double& y )
{
  return x * y;
}

class equation
{
  equation( double (&in_u)(const double&, const double&) );
//...
protected:
  double (&u)(const double&, const double&);
}

This function would be called something like $10^8$ times during a typical run.

The class goes into a library and the function u is defined by the user of the library. So I cannot have the function definition inside the class.

I have read this:

(std::function) ... has the disadvantage of introducing some (very small) overhead when being called (so in a very performance-critical situation it might be a problem but in most it should not)

Are there any more efficient ways of passing the function u to the class equation? And would this count as "a very performance-critical situation"?