It's completely dependent on the requirements of the product you are building. If code performance is a requirement (or a known problem), than you should use a

It's completely dependent on the requirements of the product you are building. If code performance is a requirement (or a known problem), than you should use a tool (ie, a language) that solves it. For most web / mobile startups, this is usually not the case, and they opt for the language which they are most familiar with, easiest to hire for and allows them to iterate and pivot the fastest.