Try loading up an image on a different instance, or browsing that instance from a different instance. If you can figure it out, it's as slow as molasses.
Looking at the image source despite it being uploaded to another instance it's hosted on Fosstodon: https://cdn.fosstodon.org/cache/media_attachments/files/110/...
That said when I see posts linked to Mastodon on HN I do notice that maybe the HN Hug of Death hits them hard as they are noticeably slower to load than my day to day interaction with Mastodon, maybe this is what you're seeing?