So true, excellent point! I just do not understand startups choosing Python/Ruby in 2022 when you can get most of the features, type safety, concurrency, async and 5 times more speed in other languages.
In addition, "real" performance is often tricky to measure and may be irrelevant compared to other parts of the system. Yes, Ruby is 10-100x slower than C. But if a user of my web service already has a latency of (say) 200ms to the server then it barely matters if the web service returns a response in 5 ms or in 0.5 ms. Similarly for rendering an email: no user will notice their email arriving half a second earlier. Similarly for a python notebook: if it takes 1 or 2 seconds to prepare some data for a GPU processing job that will take several hours, it doesn't really matter that the data preparation could have been done in 0.1 seconds instead if it had been done in Rust.
Especially for startups where often you're not sure if you're building the right thing in the first place, a big ecosystem of prebuilt libraries is super important. If it turns out people actually want to buy what you've made in sufficient numbers that the inefficiency of Ruby/Python/JS/etc becomes a problem then you can always rewrite the most CPU intensive parts in another language. Most startup code will never have the problem of "too many users" though, so it makes no sense to optimize for that from the start.
The productivity gain is so great it outweights everything else. It's as simple as that.
Developer salaries are way higher than CPU and server costs. Productivity wins out here vs. performance.