Is Ethernet traffic slowing OS X? Running OS X 11 or 12 on late 2012 iMac connected to company LAN via 1000BaseT.  Even when not running browser, email client, …, that would use the network, Mac is quite slow. RAM pressure is light (8 GB of 32 used), negligible disc or other I/O, CPU 80% idle. Worst during peak work hours, better nights and weekends.  Disconnect the Ethernet, and performance improves to level to which I'm accustomed.

Firewall is on with strict settings.

Does the network interface require bus or CPU to handle traffic not addressed to me?  Broadcast flood?

I tried sniffing with WireShark, but it was like trying to drink from a firehose.

Ideas?
If you're connected to a switch then it should be filtering the traffic for you, other than broadcast data as you mentioned. If you're using a dumb hub then all traffic would be sent to all ports. In fact, pretty sure WiFi is like a dumb hub- it sends all traffic and all clients are listening.