Have written the following code:

Code :
        glPixelTransferi(GL_MAP_STENCIL,TRUE);
 
        long timeBeforeDrawPixels=timeGetTime();
 
        glDrawPixels(FINAL_WIDTH,FINAL_HEIGHT,GL_STENCIL_INDEX,GL_UNSIGNED_BYTE,pboImageReadBackRed);
        //FINAL_WIDTH and FINAL_HEIGHT are set to 1024 and 768 respectively
 
        long timeAfterDrawPixels=timeGetTime();
 
        printf("We found this artificial noise lasted for %ld miliseconds.\n", timeAfterDrawPixels-timeBeforeDrawPixels);
        //I got above 400 ms here on GeForce GTX 650 Ti.
 
        glPixelTransferi(GL_MAP_STENCIL,FALSE);

Can anyone give any hint on this issue? Is it a driver error or architecture error? Thanks in advance!