Ok i just gave this a try on a board that has a lot of forums (500+) where the forumhome is loading kinda slow (5-10 seconds to fully load). In the top of the page there are 2
animated banner images.
I do understand that is only a visual trick and doesn't really speed things up.
How i tested?
- Applied the above change
- Did an initial pageload (i asume it doesn't make a difference on the first page load)
- Scrolled to the bottom of the page
- Did a page refresh
This was really a confusing

experience.
- To my feeling it took a bit longer until the cursor changed to the 'busy' icon.
- While still viewing the bottom of the page (where i ofcourse can not click anything, since the page is not really loaded yet), the 2 dynamic banner images where showing on top of the text in the bottom of the page.
- After the normal time it takes to load the page, the screen seems 'suddenly' to refresh, the 2 banner images that where blended over the bottom of the page disappeared, and normal control was returned.
Bottom line i think is that while it might give a better look on some forums, it might also be causing confusion on others. So use it with care.
If you have a relative fast loading page (from a human perspective, loading in less then 1-2 seconds), it might give the human eye the impression of faster loading. If the page takes longer, then it will probably be confusing.