That may be true for very memory constrained platforms like Heroku, but is a non issue at larger scale. Taking your figure of 300MiB per process, and assuming

That may be true for very memory constrained platforms like Heroku, but is a non issue at larger scale. Taking your figure of 300MiB per process, and assuming you might want to run 1.5 process per core to account for IO-wait, that's 450MiB per core (ignoring copy-on-write). If you look at various hosting offering, most offer something like 4GiB of RAM per core (e.g. EC2 "general purpose" instances). As for the lack of concurrency support, Active Record has asynchronous queries since a few versions, and is now even async/fiber compatible. So if you truly have an app that would benefit from more concurrency, you can do it.