Lag in HTML5?

Does anyone else experience a significant timeline lag when playing a course in HTML5? My course includes layers that each have a sound file with text and state changes timed to match. The sound plays immediately, but the rest of the timeline lags distractingly. I know HTML5 is far from perfect, but are there any suggestions out there to build in some type of load time or make this match up a little better? Could delaying the sound help?

I'm having better luck deploying in the mobile player (though I've written about a few issues there as well), but my boss would really like to have something that would play decently in HTML5. I realize that my course is fairly large-- is there a size/design structure that's better guaranteed to work well?

The HTML5 version of my course is here: http://assets.gcflearnfree.org/testing/html5compliant/story_html5.html    

Thanks!