Hi there
600 seconds? Seriously? OMG, that's ten minutes!
Personally, I would never recommend having a slide longer
than maybe ten
seconds. I'm not sure how you even manage a timeline that
long.
At any rate, whatever floats your boat I suppose. My guess is
that you have some object (Click Box, Button or Text Entry Box)
that is sitting on the timeline and pausing things before the
button you wish to see has a chance to appear.
Cheers... Rick