It sounds like you're undervaluing concurrency. Concurrency is huge for a web application. If you were say doing image manipulation or other DSP where you neede

It sounds like you're undervaluing concurrency. Concurrency is huge for a web application. If you were say doing image manipulation or other DSP where you needed "raw speed" you'd want to choose a language like C, Go, Rust. But in a web application you're handling hundreds, thousands, to millions of requests per second; concurrency is crucial for that kind of throughput. A developer on my team did some quick benchmarks and found that that Phoenix gives an order of magnitude better performance than Rails. Thats not negligible.