FR is a very old web design that doesn’t use AJAX (http://en.wikipedia.org/wiki/Ajax_%28programming%29). People are always reloading for the latest update and instead of fetching just the update, the entire page must be loaded. Unlike other popular conservative forums, FR allows links to images so this is another major factor.
Now they tell me. Face-palm. Geez, they should told me about that fifteen years ago.
Links to images are no factor at all.
When someone puts a link to an image in their posting, it's to another server -- either the original source, or photobucket/imageshack,etc.
When you view that posting, your browser goes and gets the image from the other server. FR servers aren't involved at all, unless they happen to be serving the image.
It is very, very rare that a linked image slows a page load here.