Sadly, there's no setting to change this unless you're using the iSynth viewer on the iPhone.
Pressing the spacebar plays the same order as the slideshow, so you could manually press the spacebar every five or ten seconds if the default slideshow is too fast for your internet connection or your personal preference.
For that matter, I'm sure that there are programs which would send a spacebar tap every so often for you automatically and you could use that to get around the lack of settings on the viewer. (I haven't looked into that sort of program before so I can't reccomend any particular programs, but a quick two minute search yielded this: http://www.autohotkey.com/ which is the sort of thing that I'm talking about. I haven't tried it so I don't know how user friendly it is, but it should get you looking in the right direction. I'll post back if I find anything better, or you could share a link if you find a better program.)