On a recent project I tried changing the background effect of all my slides (adding a 'dark shadow border' (#4 on most slides).
As soon as I did that it caused all the preview of each slide to be significantly slower, which threw off the timing of all the slides with that change.
It didn't effect the export/rendering, as that rendered with the original timing.
So it appears that there is a problem with the preview workflow and trying to render those background effects on-the-fly.
I'd suggest possibly just not rendering those background effects during preview output.
For now, I had to just skip using the Background effect on any projects that are sync'd to a narration track, because it's basically impossible to figure it out (without doing many exports/renders and adjusting each time in between).
Thank you for your post!
Version 1.05 fixes all these issues. You just need to refresh the slides by clicking on each thumbnail on the bottom part and make sure you save the changes first before doing the preview. Please let us know if this helps.