Basically every time you open a page (same thread, new thread, other...) you take a little bit of data from the server which holds OWTB and move it to your pc/phone/device. This in isolation is fine, but we have a very active and quite large user base which we're really proud of building up (for the fan base/club size). So if there's one thread on a topic you pull that data back to you once. If you have to go to three threads for the data you're pulling it back three times, and thus tripling the cost. Again, in isolation that's a tiny amount, probably (not really measurable in this way) 0.001p or something tiny. But when that's a lot of people it all adds up.
This is a pretty simplified example, but hopefully it helps.
Appreciate the ideas, but unfortunately they don't offer much in the way of savings because, as above, it's more about the active threads and traffic than the things sitting on the site in the background. We have talked about reducing the open forums and may yet do that anyway, but it is unlikely to offer a significant saving.
Haha! I probably should move all this to the main begging thread. I will try and get time later.