Why isn't there a fast blockchain update inside a LAN behind a router between two Bitcoin clients? This is the scenario:


2 computers on a LAN with private IPs (192.168.X.X)
router forwards port 8333 only to one of them (the 2nd one).
one computer has the full blockchain updated in its Bitcoin client, 
the 2nd computer has only half the size of the block chain (Bitcoin client has not been run for a long time).


Why doesn't the Bitcoin client from the second computer detect the first computer and receive the remaining missing blocks at high LAN speed?

What i see is a very slow update of the chain on the 2nd computer.
Because the bitcoin client tries to avoid being "fenced in" by an attacker who wants to control all the information flowing to it. It does this by connecting to more distant nodes, i.e. nodes which don't share the first two octets of the IP address. (update: the new address manager code still defends against this type of attack, but probably not in exactly the same way any more.)

If you know of a trusted nearby node, you can configure it with the -connectcommand line option or the connect= setting in bitcoin.conf.