Actually my understanding is a bit different. Flannel uses VxLan and acts as a overlay network (packet inside of packet) while Calico is a pure Layer 3 routing

Actually my understanding is a bit different. Flannel uses VxLan and acts as a overlay network (packet inside of packet) while Calico is a pure Layer 3 routing solution. This means some performance gains but less protocol support vs VxLan. I think it would be quite interesting to test