Well, I've been noticing as of late when people try to use the resize command for the forums to make the images 800x600 when they are originally bigger, the forum lags bad when you try to scroll past them.
One thread here that I recently viewed, had one of those such images, plus it was over 2MB size in size, and that page of the forum lagged really bad when trying to go past it. It was that noticeable.