If you want to use Google Slides to make an Ignite-style presentation where the 20 slides are each 15 seconds, throw this animated GIF in the corner of the master. Want to make your own? Check out the repository and customize it.
There are 20 copies of the 15-second sequence in the GIF. This is for two reasons. First, Slides doesn’t reload the GIF between transitions, so if you had a single 15-second non-looping GIF, it would halt after the first slide. Using a non-looping GIF lets you add some slide-to-slide animations, such as a yellow progress bar telling you how far you are in the presentation.