The LR manual (http://helpx.adobe.com/pdf/lightroom_...) says:
The playback settings don’t apply to exported PDF slide shows. The slide duration and fade transition are fixed in PDF slide shows.
However, this appreas to be incorrect. In fact in my version of LR5 RC5, the slide duration of exported PDF slideshows does follow the setting of the slider even when set to manual.
I would appreciate to be able to set the transitions to "manual" (I believe this was the case in LR4 but I can't check this anymore).
If this is not possible, extending the selectable range to 99 seconds makting the PDF slidshow to run at 99 seconds if "manual" is ticked would be an acceptable option.