This should be in the settings program, but at present there is no way to enter numbers. There are plans to rework the the settings and that will no doubt include data entry.

Until then we could add a feature to rotate through a number of pre selected speeds. Can you suggest some suitable value? 2 or 3 values tops would be best.

There is currently a way to do this that involves editing a file: You need to make a simple change to this file:

...\MaavisPortable\App\extension\chrome\content\maavis\slideshow.xul

At line 41 (near the end)

<slideshow id="player" row='1' col='0' rows='8' cols='7' captionId="caption" />

add a rate='N', where N is number of seconds you want each
slide to display for. e.g

<slideshow rate="10" id="player" row='1' col='0' rows='8' cols='7' captionId="caption" />