I admit I am surprised by how much a difference there is, it should be just Tread and Rread headers shouldn't it?
If you have high latency or high bandwidth, the maximum message size for the 9p messages will be too small to keep the pipe full if you're using read serially. Did you take a look at how much bandwith was actually in use during your tests?
-Steve
Tim Newsham | www.thenewsh.com/~newsham | thenewsh.blogspot.com