My 78 image slideshow is perfect when exported to PDF slideshow but when exported to 720P MP4 the image quality is very poor when viewing on my high-end desktop computer. I'm guessing that the resulting 568MB file has been achieved using an unstated compression rate so as to fit onto a CD. If this is the case then I would prefer an option to have a larger file so I could save it to DVD.
Hope someone can respond.
720p is a very low resolution and will generally look poor on a high end machine with a high res display simply because you have very few pixels. You should at least do 1080p. The compression that Lightroom uses is EXTREMELY inefficient. It is highly surprising how bad the compression is that it uses. You will always gain enormously in space without losing quality by running the resulting file through media encoder (if you have the full CC) or through the free and open source handbrake.