Julia not 220 times faster than C++

General Usage
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9

My latest CPP code:

#include <iostream>
#include <Eigen/Dense>
#include <chrono>

using namespace std;
using Eigen::Vector3d;
using namespace std::chrono;

struct KPS {
    Vector3d v_wind_tether;
    Vector3d v_apparent;
    Vector3d last_tether_drag;
} ;

KPS kps;

double calc_drag(KPS& s, Vector3d& v_segment, Vector3d& unit_vector, double rho, Vector3d& v_app_perp, double area)
{
    s.v_apparent = s.v_wind_tether - v_segment;
    double v_app_norm = s.v_apparent.norm();
    v_app_perp = s.v_apparent - s.v_apparent.dot(unit_vector) * unit_vector;
    s.last_tether_drag = -0.5 * rho * v_app_perp.norm() * area * v_app_perp;
    return v_app_norm;
}

int main()
{
    // std::cout << kps.v_wind_tether << std::endl;
    Vector3d v_segment(0.1,0.2,0.3); Vector3d unit_vector(0.0,1,0); double rho = 1.0;
    Vector3d v_app_perp(10,0,0); double area = 0.3;
    double v_app_norm;
    auto start = high_resolution_clock::now();
    for(int i=0; i < 100000; i++) {
       v_app_norm = calc_drag(kps, v_segment, unit_vector, rho, v_app_perp, area);
    }
    auto stop = high_resolution_clock::now();
    cout << "calc_drag needs "
         << ((stop - start).count())/100000
         << " ns." << endl;
}

And my CMakeLists.txt:

cmake_minimum_required(VERSION 3.0)

project(Tutorial)

find_package (Eigen3 3.3 REQUIRED NO_MODULE)

add_executable(${PROJECT_NAME} "main.cpp")
target_link_libraries (${PROJECT_NAME} Eigen3::Eigen)

if(NOT CMAKE_BUILD_TYPE)
  set(CMAKE_BUILD_TYPE Release)
endif()

set(CMAKE_CXX_FLAGS "-Wall -Wextra")
set(CMAKE_CXX_FLAGS_DEBUG "-g")
set(CMAKE_CXX_FLAGS_RELEASE "-O2")

Which gives an execution time of 5ns with -O2 optimization level, the same level I used for Julia.

But measuring these very short times is tricky on C++.

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