If you're moving those kind of speeds you're probably not doing packet filtering in software.
But you may be using Unix-y software to manage the interfaces and do offload programming:
* https://en.wikipedia.org/wiki/Vector_Packet_Processing
* https://www.youtube.com/watch?v=ptm9h-Lf0gg ("VPP: A 1Tbps+ router with a single IPv4 address")
I use VPP and handle bonded speeds of 200gbit. Not that far fetched to also do this at 1000gbit.
Probably? But if you are then you’re certainly not using OpenBSD.