The SlidePanel is kind of a convenience control. It convenient because it will make a slide show out of whatever top level elements are inside the panel. There is a tradeoff between convenience and optimization, an optimal solution for a slide show would not be as convenient and not as easy to use and would require custom javascript.
The issue is that the javascript won't run until after everything in the page is loaded, and since the javascript is what hides the images and turns them into slides then you can briefly see the images before that happens. So if the page has a lot of images it has to load all of those before the javascript for the slide show ever runs. The only optimizations for SlidePanel are to first optimize the images themselves so that they are as small as possible so they load faster, and use fewer of them to reduce the number of http requests needed to load the whole page.
For a custom solution there are ways of loading the images directly from javascript rather than embedding them as markup with <img src=... which means they have to load first. But again the downside of that is you have to write more of the javascript yourself to accomplish a more optimal solution. You can always implement your own custom solution as a UserControll and then embed that inside layout.master instead of using SlidePanel.
Hope that helps,
Joe