Could you please compare it with kore [https://github.com/jorisvink/kore] which is another C web server that seam quite efficient when tested with the benchmarking tool wrk [https://github.com/wg/wrk]. Result are bad with the benchmark tool Siege for kore because of its particular request pattern.
You might find this set of benchmarks inspirational [https://github.com/nanoant/WebFrameworkBenchmark]. The Java web server seam faster because it returns less data. kore is faster in fact.
Note: vibe.d has improved its performance with more recent versions. The author of the benchmarks ignore requests to update the benchmarks.