You must be joking. A good Rails dev will run absolute rings around any java or .NET effort no matter what tools they're using. I don't even think I'm being subjective here; I work with java (well, kotlin/scala) devs and even they wouldn't say anything like that.

Rails has disadvantages yes, concurrency and its lack of ability to support long-running tasks foremost amongst them, IMO. But dev speed, for standard "startup web app" features? There is no contest and I'm baffled as to why you would claim otherwise.