ZeroMQ: Why was a ZeroMQ test on IPC transport-class slower than on TCP loopback endpoint?

I performed the same test with same data size and same number of messages once with IPC endpoint and once with TCP loopback address and I saw a significant difference in the time for the data transport.
1) Isn't TCP supposed to be slower because of the network stack?
2) Does this have to do with pyzmq library?