Droplets on digital ocean running docker containers. That’s as portable as it gets, but it’s also very DIY. A lot of the reasons you might use a cloud provider is the wide host of services that are meant to make your life easier.
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.