Slideshows are high on my needs list.
One trick I have used to reduce size is reusing a silent sound reel for all slides. This is for a slideshow with no fades, reel by content, thought to have a really big slideshow that can have VF files delivered for different versions during the festival. It can apply in other situations to preshow quiz, etc.

My thoughts on this:
-Short reel size: Reusing a 1 frame reel will probably blow up some server somewhere. A 1 second reel may help and sounds less scary. Doremi specifies a minimum 4 seconds for playlist content, of course reel size is different and probably more resilient to shorter lengths.
-Asset chunks: Always been curious to try if it's possible to use chunks on the assetmap as an equivalent of reels, and if servers will treat them differently, and probably chunks at a previous and more efficient level.
-Low framerate: 1 fps? 15 fps archive standard?
-(THIS IS THE ONE) Image based subtitles: The slides would be PNG full frame size "subtitles". A black image reel is required but very light. For silent slideshow, both the image and silent sound reels can be small ones reused. For music, a long black can be created with a small file size, very raisonnable too. Image subtitles officially support fades and all!