Performance on local area network is 5 times worse than on local machine

Just tested couchbase(1.8.1) on two different network environment, but performance differ so much:

two machines are the same and in a local area network(100M), bose 4G ram, i5 cpu , ubuntu 12.04, bucket is 2G Mem , couchbase, no replica, Cluster is on machine A,
then test write on machine B (base on c client library), and connect to A for writing, i got result like this : a writing operation cost 400us (size is 128Byte per operation), However , when i run the same testcase on machine A and connect to cluster on A , result is 80us, so the result get 5 times better.
And then I install couchbase server on machine B and the result is still the same (5 times differ). I am assured that the network is ok, and i can do scp between A and B at an average rate of 11M/s.
So I wander if my test result is right? Does couchbase perform worse so much in a local area network?