This very much depends on the use case. If you are truly CPU-bound than yes, ruby is a bad choice. Similarly, if you are IO-bound or otherwise need a lot of concurrency ruby is not great. I also think it's problematic for large code bases with hundreds of developers. However, there is a huge swath of web apps and APIs that just need to pull a bit of data from a DB or two, munge it together and send it to the client. Rails is perfectly horizontally scalable and plenty fast for this even at scale given Amdahl's law.

That said, there are definitely some self-inflicted wounds people run into here. Of course there are the oft-cited expert beginner problems like n+1 queries and missing indices, but there's a more subtle issue as well: ActiveRecord itself is so convenient that it discourages writing baseline efficient code even when you need it. Basically any time you need to process a medium to large amount of data, ActiveRecord acts like a sort of super boxed type (to use Java terminology) adding a huge amount of overhead. Yes it's true that ruby doesn't even have true primitive types that can achieve the performance of Go or Java, but often times ruby primitives are plenty fast, you just have to be willing to write a bit of extra code, and ActiveRecord as an ORM has plenty of escape hatches at various levels of granularity to facilitate this (eg. pluck, find_by_sql, update_all, execute, etc).