libthr:

同時10接続:
Requests per second: 6698.82 [#/sec]
Requests per second: 6330.32 [#/sec]
Requests per second: 6437.08 [#/sec]

同時50接続:
Requests per second: 6292.87 [#/sec]
Requests per second: 6405.33 [#/sec]
Requests per second: 6567.71 [#/sec]

同時100接続:
Requests per second: 6121.82 [#/sec]
Requests per second: 6424.67 [#/sec]
Requests per second: 6439.56 [#/sec]

同時200接続:
Requests per second: 5997.00 [#/sec]
Requests per second: 6238.30 [#/sec]
Requests per second: 6237.52 [#/sec]

同時500接続:
Requests per second: 5912.96 [#/sec]
Requests per second: 6272.74 [#/sec]
Requests per second: 6298.02 [#/sec]

同時1000接続:
Requests per second: 5775.01 [#/sec]
Requests per second: 6248.83 [#/sec]
Requests per second: 6208.48 [#/sec]

感想:
接続数が増えるとlibthrの圧勝。ほぼ性能低下なし。