Which is fine, as long as one doesn't display links to a lot of different users on one page (like an avatar alongside comments, hosted on the <user>.site.tld do

Which is fine, as long as one doesn't display links to a lot of different users on one page (like an avatar alongside comments, hosted on the <user>.site.tld domain). As long as all those different user domains don't appear on one page, there's nothing to prefetch...