Overlay network with only 22,8 kB/s. What? Why?

Hi. I’m running several CentOS7 on Azure with Docker 1.10. Created an overlay network in my Docker Swarm w/ TLS. No problems at all. However during speed tests with Containers attached to the overlay network I see networks speeds of only about 22 kB/s. No network drops or errors are reported through ifconfig. When I start the very same containers (Ubuntu) using the default bridged mode of Docker I get the usual readout in the MB/s range again. There is a VPN gateway configured in the same Azure subnet but apart from that its just usual Linux VMs configured through Azure Classic deployment mode. Anything that I missed? If I’m starting both containers on the same host, still using the overlay network I get normal speed again. Start on different Docker nodes -> Speed goes into kB/s range again. I’m really puzzled on what could cause this…