probably what’s happening is that your ISP is throttling or blocking your connection to the host (https://inaturalist-open-data.s3.amazonaws.com/) where some of the photos in iNaturalist are stored (specifically, the photos that are licensed, as opposed to having all rights reserved). you’ll probably have to call your ISP to work out the problem.
here’s another example of something similar: https://forum.inaturalist.org/t/cc-licensed-images-not-loading-and-then-painfully-slowly-in-south-africa/35994/7.