China’s ByteDance, the world’s most valuable startup, had its first success with a personalized news feed, Toutiao. New data shows its short-form video app Douyin has become an even bigger hit. But there are signs that the rapid growth rate of Chinese video apps is starting to slow. And that raises a new question for ByteDance: Does it need to come up with yet another new blockbuster to keep up with investors’ rising expectations? ByteDance investors—including Sequoia Capital China, General Atlantic and SoftBank—have a lot riding on that question.