如果我用代码块编译代码，为什么我的代码运行速度更快

我想测量相机的FPS。我在这里找到了一个简单的代码。如果我用CodeBlocks（在Ununtu上）编译代码并运行循环600次，结果为27 fps。

如果我从终端进行编译：

    g++ -Wall main.cpp -o  main -I/usr/local/include/ -lopencv_core -lopencv_highgui

结果是14 fps。为什么从终端编译后会这么慢？

这是代码

#include "opencv2/opencv.hpp"
#include <time.h>
using namespace cv;
using namespace std;
int main(int argc, char** argv)
{
// Start default camera
VideoCapture video(0);
// With webcam get(CV_CAP_PROP_FPS) does not work.
// Let's see for ourselves.
double fps;

// Number of frames to capture
int num_frames = 600;
// Start and end times
time_t start, end;
// Variable for storing video frames
Mat frame;
cout << "Capturing " << num_frames << " frames" << endl ;
// Start time
time(&start);
// Grab a few frames
for(int i = 0; i < num_frames; i++)
{
    video >> frame;
}
// End Time
time(&end);
// Time elapsed
double seconds = difftime (end, start);
cout << "Time taken : " << seconds << " seconds" << endl;
// Calculate frames per second
fps  = num_frames / seconds;
cout << "Estimated frames per second : " << fps << endl;
// Release video
video.release();
return 0;
}