database read replicas, when do they actually help?

Lately, our main database has been slowing down during busy times, so we're thinking about adding read replicas. I know the idea is to handle more read traffic, but what are the actual downsides? How much delay do we get with replication lag? Also, are there certain kinds of apps or queries that really benefit from read replicas, and others where they just make things more complicated than they're worth? Just wondering what experiences others have had.