Is Bonus Round on a Timer before it starts?

Does the bonus round check how long the game has actually been played or does it always start extremely hard 1 second in? My creeps had millions of health at the start of the bonus round, is this intended? I was using a Balou strategy with a ton of beavers to increase the time in between rounds so that Balou would get higher damage. However at the start of the bonus round Balou could barely keep up with the first wave. This can't have been intended? If this was intended then Balou is pretty worthless for a Purple Tower Card.