I have a program where I read a handful of tiny images (50000 images of size 32x32). I read them using OpenCV imread function, in a program like this:

std::vector<std::string> imageList; // is initialized with full path to the 50K images
for(string s : imageList) {
  cv::Mat m = cv::imread(s);
}

• Sometimes, it will read the images in a few seconds.
• Sometimes, it takes a few minutes to do so.
• I run this program in GDB, with a breakpoint further away than the loop for reading images so it's not because I'm stuck in a breakpoint.
• The same "erratic" behaviour happens when I run the program out of GDB.
• The same "erratic" behaviour happens with program compiled with/without optimisation
• The same "erratic" behaviour happens while I have or not other programs running in background
• The images are always at the same place in the hard drive of my machine.
• I run the program on a Linux Suse distrib, compiled with gcc.

So I am wondering what could affect the time of reading the images that much?