Your switch is causing slow traffic? Yeah never seen such a thing other than duplex mismatch or lower speed.. You sure your not limiting traffic on the switch? That is something you could do.. But then say iperf between pfsense and client through the switch gives you 850?
Did you do that iperf test udp or tcp?
I have been working with cisco switches and routers for years and years and years… Never seen such a thing.. What is the config you have on the ports? Just post show run interface.. Then again we don't use EOL hardware that got off ebay ;) But if your saying you are seeing full speed between the devices across the switch, but not to the internet that makes zero sense.