Don't know much about how the internals of PfSense are built, but I do know UDP Video quite well. I assume this is UDP video traffic ?? FYI, there's plenty of mid-range switches out there from well-known high end vendors that also have issues with UDP streams - mostly those that implement the switch / routing logic in SW rather than HW.
Most likely it's the way the graph data is captured doesn't accurately reflect the outbound NIC buffers. At 60Mbps, they should be full most of the time and streaming close to the source rate. Also what's the line rate on the two ports, and what other traffic do you have on there ?
First thing would be to capture the inbound & outbound traffic for 30 seconds or so using tcpdump. Then analyse the files for timing in Wireshark, which will give a better idea of the actual flows.