There is something wrong with your instance, lemmy.today. i found the post you had problems with. The post uses the raw image as its thumbnail which is 8MB. 8 MB for an image is considered very large. 8MB for a thumbnail is considered extremely large. This is also likely why images appear to load very slowly for you. In comparison, on lemmy.world, the thumbnail image is just 800kb.

When you are looking at the post feed, the app needs to load several thumbnails at once. If these thumbnails are all very large it increases the likelihood of errors, either network errors or memory errors.

500 errors are typically not a problem with the app but with the server.

If you have a link to the post/comment, i can doible check.