The higher quality your images have, the heavier the files would be. When you have lots of big and heavy image files on your website, it would affect your page speed.
You could speed up the loading of your website by slightly reducing your image quality. In Portfoliobox, you can adjust the image quality by clicking on the Cogwheel Icon > Settings > General > Website > Image Quality. The image quality scale is from -1 to 1. -1 is the lowest and 1 is the highest. You can try out different quality level to find the best option for your website.