C++11 auto is much faster than std::function.

ChronoHelper.h

#pragma once

#include <chrono>
using namespace std::chrono;

inline double get_time_sec(void) {
	return static_cast<double>(duration_cast<nanoseconds>(steady_clock::now().time_since_epoch()).count()) / 1000000000;
}

// ref: [(c++11)chronoを用いた時間計測(タイムスタンプのとり方) - Qiita](https://qiita.com/hurou927/items/a2d63837e731713c7a22)

main.cpp

#include <iostream>
#include "ChronoHelper.h"
#include "MyTest.h"

int main()
{
	// Result:           x64     x64     x86
	//      loop count    1B      4B     4B
	//   local       : 0.28s   1.32s   1.57s
	//   template    : 0.33s   1.32s   1.50s
	//   std:function: 2.18s   9.11s  12.17s
	
	// Compiler: VS2017 15.8.6 (C++17)
	// OS: Windows10 Oct2018Update

	unsigned long long sum = 0;

	// local
	std::cout << "Loop(local)" << std::endl;
	double start = get_time_sec();
	for (unsigned int i = 0; i < CMyTest::LOOP_COUNT; i++) {
		sum += i;
	}
	double end = get_time_sec();
	//std::cout << sum << std::endl;
	std::cout << "elapsed: " << end - start << "sec" << std::endl;

	// use template
	sum = 0;
	std::cout << "LoopByTemplate" << std::endl;
	start = get_time_sec();
	CMyTest::LoopByTemplate([&](auto&& v) {
		sum += v;
	});
	end = get_time_sec();
	//std::cout << sum << std::endl;
	std::cout << "elapsed: " << end - start << "sec" << std::endl;

	// use std::function
	sum = 0;
	std::cout << "LoopByFunc" << std::endl;
	start = get_time_sec();
	CMyTest::LoopByFunc([&](auto&& v) {
		sum += v;
	});
	end = get_time_sec();
	//std::cout << sum << std::endl;
	std::cout << "elapsed: " << end - start << "sec" << std::endl;
}

// ref: [C++11 autoはstd::functionより高速 – Hossy](http://www.ngc.is.ritsumei.ac.jp/~hossy/2015/07/c11-auto%E3%81%AFstdfunction%E3%82%88%E3%82%8A%E9%AB%98%E9%80%9F/)

MyTest.h

#pragma once
#include <functional>

class CMyTest {
public:
	static constexpr unsigned int LOOP_COUNT = 4'000'000'000;

public:
	static void LoopByFunc(const std::function<void(unsigned int)>& func) {
		for (unsigned int i = 0; i < LOOP_COUNT; i++) {
			func(i);
		}
	}

	template<typename TFunc>
	static void LoopByTemplate(const TFunc& func) {
		for (unsigned int i = 0; i < LOOP_COUNT; i++) {
			func(i);
		}
	}
};