I see you’re using full image size. It’s recommended that you use a smaller image size for the grid display and only use the full size for lightbox display.
As you can see in the screenshot below, your image size is 1538 x 2048px but to grid display only requires 180 x 240.
Grid Settings > Media, try using Medium for the Image Size.
If you haven’t done so yet, you should also optimize your images.
Image Compression is a crucial thing to do but most people forget it and then are wondering why their site loading speed is very slow. Images are one of the biggest part of your website. An image heavy site can still be quick if you watch out for some things which we will cover in a second.
First of all always try to use JPEG or GIFs rather than PNGs since they are smaller in size. A lot of people upload their big images and WordPress will crop them in all the different thumbnail sizes and you will insert the image with the proper thumbnail size in the post and then people think it’s all set.
But the most important thing to do is compressing your images otherwise you will have big images and a slow loading time. There are different ways on how to compress images. Below are a couple of services and WordPress plugins you can use to optimize your images for decreased loading times.
WP Smush Pro (WPMU DEV). Smush Pro is a cloud based image optimizing plugin, that works on a API based system, that allows you with a few clicks to send your images for optimizing and also includes loesless compression with it’s Super Smush option.
is another cloud based image optimization service, the main difference being, they not only have a WordPress plugin you can use, but you can also optimize images directly on their website, rather than using the plugin. Kraken.io
Ewww Image optimizer is another cloud based image optiomization plugin, that also allows you to optimize and compress images to decrease load times.
If you are creating images in Photoshop a common thing that people do wrong is save the images without using the option “Save for Web & Devices”. This is very crucial since you can set there the compression and quality rate and other things which will lead to a smaller image size. Other image editing programs can do the same so always look in the documentation for image compression.
Hope that helps.