One problem I have with the slideshow in impress, is that it waits to display the current slide before being able to move to the previous or next slides. This is a real problem if you have big images which take time to be displayed (see bug 83426). If you want to move by several slides, it will take a lot of time.
I propose to have an asynchronous mechanism: you can always change the slides, and they are displayed later if time allows.
Not sure of the best way to do this. Obviously it is less of an issue for slides of simple composition. But becomes more of an issue for slides with complex images that have to be fully rendered prior to preview being available.