Without having access to your raw data and configuration for the server and web it’s difficult to tell what the problem is. Has it always been this slow, or has the problem appeared over time? Is it all images, or just some of them. I noticed your OMERO hasn’t been upgrade to 5.6 yet.
Since these are public images one option you could investigate is to enable caching in Nginx (see https://www.nginx.com/blog/nginx-caching-guide/ for an introduction) which is what we use in the IDR. You’ll have to be careful though to prevent private data also being cached, for instance you’ll need to disable caching of cookies, certain headers, and you’ll also want to ensure caching is only enabled for particular image IDs.
If you give us more information we might be able to suggest other things to try, though they may assume OMERO 5.6.0/Python 3.