[spullara]

Just depends entirely on what you are doing. If you are ecommerce you can basically stick to really slow languages forever as the request volume is so small it should be a small fraction of your application. For consumer scale you end up scaling with caching and eventually migrate or severely optimize - like writing your own language runtime like FB. None of this accounts for all the analytics they are doing which is mostly written in Java and C++.

[donw]

Even outside of e-commerce -- I would say anything on the web.

Run something like Ruby/Python for the majority of your web services, and deploy Elixir/Go/Kotlin/C++/Swift whenever you hit a data-munching need that Ruby/Python can't handle.

This doesn't fly if you're doing games, or augmented reality (AR), or mobile apps, or embedded stuff... but the web is still a pretty big place to build.

[Cthulhu_]

Pretty much. Solve the problem first, then measure where the bottlenecks are and focus on those.

The premature optimization adage has been touted around since forever but it can be widely applied.