I've searched for keep-alive issues, but there is none, so might still not be fixed since my Linux distribution version httperf 0.9.1...

Please have a look at my comment here...
rwf2/Rocket#2062 (comment)

To summarize: In httperf it depends on the receive-buffer size how and when received data segments in a keep-alive session are ACKed. And mostly this is buggy and just waits around. But please see above comment for examples and explanation.