Why does it takes so long to capture the frames and store them in memory?

edit

Starting with your clocks should be 1st and last instruction, we all should remember that std::clock() returns the approximate __processor time used by the process__ check the doc. If the process doesn't use the processor than std::clock() doesn't advance, this is the case when you have multi threading or a lot of I/O or sleep. In addiction std::clock() shouldn't depends so much from overall system load.

Test it with a simple std::cout << "Press a Key"; std::cin.get();

The processor time returned by std::clock() is always the same despite of needed time to press the key !!!

NOTE: THIS IS NOT TRUE ON VS 2013. see this. (Really I've same behaviour also with Win764+gcc version 5.1.0 tdm64-1)

Because it looks you are using *nix OS, in your example:

• 0.7s is the processor time consumed by your process.
• 40s is the real time elapsed between process start and stop

If you want to measure real time elapsed between 2 events please refer to so called wall clock.

Please note that you can measure time elapsed with OpenCV using cv::getTickCount() and cv::getTickFrequency()

The code below compares processor time, wall time and elapsed time measured with OpenCV

double procTime,wallTime,cvTime;

clock_t clk0,clk1;
std::chrono::high_resolution_clock::time_point chrono0,chrono1;
double cv0,cv1;

clk0 = std::clock();
chrono0 = std::chrono::high_resolution_clock::now();
cv0 = (double)cv::getTickCount();

// do something ...

clk1 = std::clock();
chrono1 = std::chrono::high_resolution_clock::now();
cv1 = (double)cv::getTickCount();

cout << endl << "Processor time (ms) "  
     << 1000.0*float( clk1 - clk0) / CLOCKS_PER_SEC;

     << endl << "Wall time (ms): "
     << std::chrono::duration<double, std::milli>(chron1 - chron0).count();

     << endl << "Wall time using OpenCV (ms):"
     << cvTime = 1000.0*(t1 - t0) / cv::getTickFrequency();
     << endl;