When does a monorepo actually slow down developers and make builds take longer?

Hey everyone, I've been thinking about monorepos a lot lately. They can make dependency management simpler and it's easier to share code. But, I've also watched them turn into huge messes. They really slow down CI/CD and local development, especially for big teams with lots of different services. Is there a point where a monorepo just isn't worth the trouble, maybe with certain project sizes, team setups, or tech stacks? I'd love to hear your stories, the good and the bad. What tells you it might be time to ditch a monorepo?