Why do my new animals take 8 hours to play 1 game for 900 stars instead of usual 2 hours?

Have your 8 hour animals “playing” while you sleep or aren’t going to be playing the game. They earn a lot more hearts or stars or binoculars. Then use your shorter time animals when you will be playing or at least accessing your phone often: You earn more per hour with the shorter time frames:

I follow the same rule using the 8-hour animals when I'm asleep or at work. I also start an 8-hour activity before hatching eggs. If you get something to level up, the "swap" gives you immediate credit for the whole 8 hours.