Most threads that carry high numbers of photos (Diogenesis' news threads for example) have their images housed on servers that are prepared for high volume (i.e., Yahoo, CNN, AP and other news agencies).
In other cases, there are images that are housed on servers that are not prepared for the kind of hammering that posting that image on FR would do. In those instances, the server housing the image is the weak link in the chain, not FR. Waiting for those images to load often is the cause behind a single thread taking an intermnible amount of time to load - and often leads to timeouts in many cases.
The situation that is happening here and now is entirely different. There is something else causing this. Whether it's a DDOS attack or a mass ping assault or a router going down, I don't know.
To get back to your question though, you're not causing a bottleneck while waiting on a thread. The content from FR passes with no problem, and in most cases, while you are waiting, your browser (much of the time) has already buffered the rest of the content from FR, and your browser/machine is waiting for the remainder of the content. It doesn't slow FR any in that event.