If you are building a SaaS company and build your site in RoR, but also have experience in say Go, and decide 'hmm instead of using RoR ill use Go for this back

If you are building a SaaS company and build your site in RoR, but also have experience in say Go, and decide 'hmm instead of using RoR ill use Go for this backend thing so its faster' That's fine. Premature optimization would be saying: I don't know Go/C++/C... but I know its fast, so instead of using what I know to get up and running quickly, i'll waste time building it in something I don't know which probably wont be as fast as doing it in something I know well. The thing is, when you're building it, no one is using it! If it's slower in RoR than Go, who cares, get it up and running, and fix it later when people are actually using it.