Wouldn't it kill the bandwidth? And I think it would affect the load time of userpages/journals as well.
Nope and sorta.
Embedded images are pulled directly from their source. If that ain't on FA, the data will not pass trough FA's servers, thus not influencing the used bandwith.
Journals etc would load as normal, but the embedded images will load after that, efficiently increasing the load time for the complete page, but not specifically the journal itself.