I don't think you can replace that message without altering the source code. But once the content has loaded, you can place a gif animation and a bespoke message on a 'loading screen', as in this example.
https://jonathan-hill.s3.eu-west-2.amazonaws.com/survival/story.html
This helps to compensate for any further 'load lag', which can sometimes happen with lots of audio or large images, depending on your user's bandwidth.
The 'Obtaining Signal Lock' screen is a slide triggered to jump to the next screen as soon as the timeline starts, but on a slow connection it remains on display until the following slides have loaded and caught up.
Hope this helps?
Also did a similar thing here, with an animation that compliments the theme of the course.
