Honestly it’s been my experience with Ruby that it’s Rails that can potentially be slow. Ruby is quite fast and even has a JIT option. Rails is by design opinionated, and for some cases I’ve found that I’ve had to work extra hard to ensure performance. That means refactoring code in slightly non traditional ways and having a deeper understanding of how Rails works under the hood (esp in the ORM). So if you think you can just use Ruby+Rails out of the box in its simplest form without experience and depth of understanding: yes it might be slow. But like with all things, you can go quite far with care and experience.
It’s not even that rails is necessarily slow, it’s more the way you use it that is slow. If you tie all your businesses logic to the database and commit complicated changes in transactions, sure it will be terribly slow.
What’s wrong with complicated (I’m not sure what that means - large numbers of rows updated? Disparate rows updated?) transactions? Depending on your RDBMS (and what it’s running on, config options, etc) this may or may not be slow.
