In a recent installation of WordPress (I tested with 3.4.1 and 3.4.2 and got the same results), w3-total-cache doesn't connect to the memcached server or create some connections but without reason stop of doing it. I did many tests with different combinations using "ab" utility and I always got the same results.
It's very strange and I suppose it's by some changes in WordPress because with an old installation (3.2.1) the plugin works really good and I get great response times with "ab".
Do you need more information to help you to debug the plugin ?
Please tell me if you need more information.