Check out this discussion of the preview DDoS issue *from 2017*.
#Mastodon devs know it's an issue. They just don't really care too much.
You also get the gem of, "I can totally see this being a problem, but I also don't see a solution to it, because people want previews."
Once again the lead dev projects his personal preferences on all users.
@volkris Yep, a major reason why Mastodon is condemned to be an unscalable toy unless these attitudes change.