[mistwang]

Looks like it was being throttled.
Please make sure "Per client limit" is configured properly for your test, when you do performance test from one client machine, you should turn off per client bandwidth and request throttling, increase the "soft" and "hard" connection limit. Those configurations are under "Server"->"security" tab.
For more information, please read
http://www.litespeedtech.com/docs/HowTo_QA.html#qa_perf

Best regards,
George Wang