c – `std :: condition_variable :: wait_for`经常调用谓词

#include <iostream>
#include <condition_variable>
#include <chrono>
#include <mutex>

int main () {
  std::mutex y;
  std::condition_variable x;
  std::unique_lock<std::mutex>lock{y};
  int i = 0;
  auto increment = [&] {++i; return false;};
  using namespace std::chrono_literals;

  //lock 5s if increment returns false
  //let's see how often was increment called?
  x.wait_for(lock,5s,increment);
  std::cout << i << std::endl;

  //compare this with a simple loop:
  //how often can my system call increment in 5s?
  auto const end = std::chrono::system_clock::now() + 5s;
  i = 0;
  while (std::chrono::system_clock::now() < end) {
    increment();
  }
  std::cout << i;
}

据我所知wait_for,在wait_for之后我应该是O(1)(让我们假设虚假解锁很少见).

但是,我明白了
对于内核4.17.14,Intel(R)Core(TM)i7-6700 CPU @ 3.40GHz,i~ = 3e8,
对于内核3.10.0,Intel(R)Xeon(R)CPU E5-2630 v4 @ 2.20GHz,i~ = 8e6.

这听起来很有趣,所以我通过比较一个运行5秒的简单循环来检查. i的结果大致相同,只有5-10％的差异.

题：
wait_for在做什么？它是否按预期工作,我只是理解cppreference错误,或者我搞砸了？

第二,(可选)问题：我的这个巨大差异来自哪里？

附加信息：
(gcc7.3,gcc8.2,clang6.0),flags：-O3 –std = c 17都可以得到可比较的结果.