I can use Ruby on Rails but I don't think an RoR app can handle thousands of comments per minute in an optimised way. I run a midsize app in Rails 5 and already having lot of scaling issues.

Iirc Reddit stopped open sourcing their code from 2017 and I guess understanding and running that 12 year old spaghetti code wasn't worth it anyway. I don't see anything useful on their github page too https://github.com/reddit

Rails will handle "thousands of comments" just fine. If you are hitting scaling issues with a midsize Rails app you probably could look into how to optimize.

Throwing Mongo at the problem is not really a solution.

GitHub and shopify are written in RoR afaik, many millions of requests go through them just fine

Its not like reddit improved since 2017... they added silver, copper and a sucky new interface but I dont think that helps anyone.

Worst possible advice, if you pardon my borderline rudeness. That's what most programmers do, following famous saying that for someone with a hammer every problem looks like a nail.

Ops problem is more architecture and scale, IMO, as opposed to what coding language or JS framework.

I’d start with Postgres on the back end and then work with scale as needed over time. The service can be written in pretty much any language like Kotlin, Java, Python, or even Rust if you want. Personally I’ve never used Swift server side.

IMO the worse option is to try to over optimize and use some shiny technology that you don’t understand how to use.

This is how we end up writing iOS apps in Kotlin despite Swift being very similar and built for iOS development.