Thought I'd have a go with bandwidthd, to keep an eye on things, but whenever I install, then copy a file from my fileserver, CPU on pfSense hits the roof even though it has no part in the traffic

Note: I am running pfSense in a VM (VMWare server) on the fileserver box, the LAN interface (that I have asked bandwidthd to monitor) is bridged straight to the internal LAN that the fileserver is using.
Apart from this small

problem, it behaves fine.