Network connectivity issues or general network slowness due to congestion can be hard to pinpoint root cause. The open-source utility Wireshark is the go to for looking under the hood of network traffic. Install Wireshark on a laptop, create a mirrored port (SPAN, RSPAN on Cisco) on the core switch. A mirrored port will see all traffic across all VLANs and allow Wireshark to promiscuously capture packets/frames and report them in a straight forward way. Communication thread(s) streams can be reviewed in isolation and multicast or flooding can easily be seen and source or destination identified for resolution.