The main problem is when the optimizations silently fail because of seemingly innocent changes and suddenly your performance tanked 10x. This is a problem with

The main problem is when the optimizations silently fail because of seemingly innocent changes and suddenly your performance tanked 10x. This is a problem with any language really (CPU cache misses are a thing afterall and many non-dynamic languages have boxed objects) but it is a much, much worse in dynamic languages like Python, JS and Ruby. Most of the time it doesn't matter, most high-throughput python code just invokes C/C++ where these concerns are not as big of a problem. Most JS code just invokes C/C++ browser DOM objects. As long as the hot-path is not in those languages you are not at such high risk of "innocent change tanked performance" Even server-side most JS/Python/Ruby code is just simple HTTP stack handlers and invoking databases and shuffling data around. And often large part of the process of handling a request (encoding JSON/XML/etc, parsing HTTP messages, etc) can be written in lower-level languages.