> how good is the latency once connections are already established, say for a real time video call over Freenet, or if this is not possible?

We're aiming for the delay between a contract's state being modified and all subscribers being notified to be no more than 1 second, which should be acceptable for applications like IM.

If you were doing something like a video call you'd negotiate it over Freenet and then establish a direct connection for the video/audio for minimal latency.

> Is there any server in the middle that all packets need to route to, especially for peers behind firewall

Freenet uses UDP hole-punching to establish direct connections between peers even if both are behind firewalls. A new peer uses a public (non-firewalled) peer to join the network initially but once joined all further communications can be between firewalled peers.