I've written a custom web server and the reason was performance. Web servers have the potential to do quite a lot and each of those activities adds latency, so

I've written a custom web server and the reason was performance. Web servers have the potential to do quite a lot and each of those activities adds latency, so it made sense to write my own web server to implement exactly what I needed and nothing more. A minimalist, custom web server can approach 10m req/s as seen on the Techempower Benchmarks.