Special effects, such as desaturation are created on the fly, without server resources. This means the browser needs some time to apply the effect on the thumbnails. It's often very fast and causes no noticeable delay on modern devices and browsers. However, when you resize the window or change the device orientation, the special effect needs to get re-applied. It's normal to see the process on older browsers or slow devices.
If you experience flashing after page load, or multiple flashes, you might have some unloadable images in the grid. Error checking removes the empty space for those images, causing neighbour thumbnails to get re-sized and the special effect re-applied on them. Check out the Some images are missing fix.