HttpClient has a poor performance with HTTP/2 get requests on Linux platforms. Benchmark is able to perform 25 requests/sec. At the same moment alternative Jetty HTTP/2 client produces up to ~9000 requests per seconds.
Similar issue is on MacOSX (490 requests per second).
There is no issue on Solaris & Windows.

Analysis has shown that the problem is TCP Nagle’s Algorithm and Delayed ACK clash. Like described here: http://www.stuartcheshire.org/papers/NagleDelayedAck/