Poor performance of sendfile in high load big file download

My server is using to download big file(10M~20M each),
when using sendfile on ,if the conn reach 800-1.2k conn/s(limit_rate
150k),
the perfmance get poor,time_starttransfer may be 20~30s.
When I change to sendfile off,output_buffer 1 512k,when conn reach
800-1.2k conn/s,
the performance is better,time_starttransfer<10s,but the NIC IO is
limit to 70-80MB/s.
My Env:
Xeon X 2(4 processce X 2),12G RAM,5 SATA as raid0
OS:AS5,Ngnix 0.8.15,
worker_processes 8;

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