I'm not sure whether you're building the slideshow manually or using the normal template.
If you're doing it manually - you're inserting the transitions - the maximum duration can only be half the length of the image before/after it... so if images A & B have durations of 10 seconds, the maximum transition time is 5 seconds.
If you're using the normal slideshow template - you clicked the slideshow button - then you've lost control of all durations, as it's built into the template. The only way of modifying that is to apply video speed later.
Cheers - Tony
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