Rather, it's the programming models put in place by the initial framework choices and architectural decisions that will make the biggest difference. Prototyping a web app in Python isn't fast because Python as a language makes it fast. It's fast when you use Django to effectively wire a database model directly to a set of CRUD endpoints in less than a dozen lines of code, complete with validation. The same can be said about Ruby + Rails.
Node could, in theory, provide a similar capability, but the platform was popularized after large sections of the developer community seemed to demonize frameworks like Django and Rails. So, whether for this reason or another, Node doesn't really have anything quite like Django or Rails, and thus would likely not match the dev speed of those frameworks, all else being equal. Python is _fast enough_, and early stage companies rightfully tend to not worry about CPU usage or hosting costs, because they don't have the scale for it to matter.
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With all that being said: pick an early tech stack that helps you optimize for iteration speed, not necessarily raw dev speed (although they often overlap). Importantly, you typically want to build a system that minimizes the cost of being wrong in a reasonable time frame. If that's Python? Awesome. If that's Node? Also awesome. If that's Java? Awesome. Pick what you/your team know best and can architect in a sane way to help you find the balance between time to market and ability to experiment.