Lazy Load Placeholders
If you don't have ExactDN, the EWWW IO plugin will use standard inline placeholders: blank 1x1 GIF images using base64 to embed them right within the page. This is the fastest method, but it isn't always the best for your visitors. The primary problem is that you end up with a bunch of blank spaces where the images will be, and if the images don't fill in quick enough, folks don't realize an image was there and they scroll right on by it.
Enter LQIP (low-quality image placeholders). Using ExactDN, we can generate a very small, blurred version of each image, so that your visitors know an image is inbound. This works great when the images are larger, but what if the original images are already pretty small? ExactDN will then return a blank placeholder image, which will be very tiny, and still save on bandwidth.
There is still one problem though, and it's something GTmetrix is good at pointing out. If you have a bunch of images that are smallish, and all the same size, you'll get LQIPs that are all identical, but served from different urls. That means you have the same exact image loading a bunch of times. While it will be fast, it's not ideal, and GTmetrix is sure to let you know if your page falls into this boat.
The solution then, if your site has lots of small images all the same size, is to force Lazy Load to use inline placeholders instead. To do that, simply add this line to your wp-config.php file (somewhere after the first line with the opening PHP tag, and before the line that says to stop editing):
define( 'EWWW_IMAGE_OPTIMIZER_USE_LQIP', false );