ofxOSC slow in broadcast mode

I am on macOS 10.12.6 with OF 0.10.1 on Xcode 9.2

I am testing to broadcast a small OSC message ( list of 40 floats) to 6 local computers.
But the data is received very slowly and seems to get buffered. This buffer takes long to get empty. Even when stopping to send data, data still drives at the receivers.

If instead I create 6 senders and send the same data one by one to each, then things run smooth on the receiver computers.

Is this a problem with my router, the 6 OSC receiver computers or … ?

This is how I setup the sender.

        ofxOscSenderSettings oscSenderSettings;
        oscSenderSettings.broadcast = true;
        oscSenderSettings.host = "10.0.1.255";
        oscSenderSettings.port = oscSendPort_mixer;
        sender_mixer.setup(oscSenderSettings);

Would it make sense decreasing the buffer size in the ofxOSCSender.cpp add-on?

static const int OUTPUT_BUFFER_SIZE = 327680;

or

if(osc::UdpSocket::GetUdpBufferSize() == 0){
	osc::UdpSocket::SetUdpBufferSize(65535);
}