I'm looking for a link, but in the meantime, I was working on a product where we had to map every connection between 2 routers to a new connection between 2 boxes (imagine having 2 boxes MITMing 2 routers and remapping every TCP session internally so you could change the traffic transparently to the recipient).

We needed to prove we could handle 100k TCP sessions and it would fail at ~64k sessions when iptables was running on either box (even with an "allow all" rule). Otherwise our hardware would fall over around the default File Descriptor limit.