ab has not been the bottleneck in the benchmarks since both servers were pinned to a single core. It is true that ab is too slow for benchmarking a HTTP server running on multiple CPU cores, but it is at least faster than a server only using one core.

Regarding the performance of the HTTP parser, I have heard that picohttpparser (the HTTP/1 parser used by H2O) is much faster than the HTTP parser used by nginx. https://github.com/kazuho/picohttpparser

If that is true, it is likely due to the difference between the approaches the parsers take. Unlike most parsers, picohttpparser does not have a callback-based API. Instead it uses a loop for parsing the HTTP request and header lines.

note: This comment might be biased since I am the author of H2O and picohttpparser.