Is the server in the university/hospital and are you testing from the same building? Try testing from your home or as fas as possible. Also use another browser, so the images are not in the cache or something.
Thank you very much for your feedback and support! I already tested it from a "remote" location.
I have optimized the code tonight and think I was able to improve performance significantly!
I am very sorry for the inconvenience - but I never expected that amount of traffic on the site, so I was a little bit overwhelmed by that :-)
But I hope that the issue is improved now!