See https://icing.github.io/mod_h2/throughput.html
This discusses the performance improvements regarding raw throughput of the current mod_http2 implementation vs. the 2.4.17 one. Some outlook onto the next release, hopefully. tl;dr h2 (https) raw transfer numbers are on par with HTTP/1.1 now, h2c (http:) are at 95% - in my setup. //Stefan PS. In case you find any mistakes or nits in there, I will hold back the tweet until this evening.
