As a result of the new image uploader, I have configured it to check the size of existing images as well as those being uploaded. This means that if you're the first person to read an older topic with lots of images, the forum will check (and resize if needed) all of the images on that page. Once they are resized, they won't need to be resized again... but this might cause a noticeable delay in viewing the page.
The reason that I'm doing this is to reduce the size of the attachment directory. We have a LOT of images on the forum and a lot of them are bigger than they need to be. As the forum automatically resizes existing images, the size of the attachment directory will shrink. And as this happens the speed at which each page loads will increase because the forum will be sending smaller images to your browser. So this is a win-win scenario for all of us.