I just bought this game a few days ago and decided to try a local network game with my brother.

(This is after downloading all the patches mind you)

I found it impossible. The game would lag horribly until eventually timing out and disconnecting one of us in the first 5 minutes of every game.

So we decided to attempt playing together online. Again the same issue. Neither of us have any firewalls but we do have a router, obviously.

We have never had any connection issues playing online with any other game(long list of games at that) or issues with port forwarding.

Anyone have any ideas what could be causing us to get disconnected and lagged to oblivion within 5 minutes of every game?

Maybe Heroes 5 Frequently Asked Questions. could help?

That was actually the first thing I read over.

Sadly no useful information for our issue was found in those 7 or so pages. Anyone else have any idea why we get disconnected from online or local games within 5 minutes of starting?

You say you don't use a firewall. Have you Windows Firewall disabled? Make sure you do.

The fact you're lagging after mins instead of immediately, indicates it is another process interfering. Make sure you both boot Windows up without any unneeded programs running on the background (tick off all items in msconfig > startup items).
Of course you should try to connect both PC's directly (with a crosscable, buy it in a shop) instead of the router, setup a new connection, and then try again.

I am having the same problem Mongoose. However, we used to play on the home network with HOMM V, but I think it was after patch 1.04 that our joyful multiplayer LAN went to the dogs.
We can play online to Battlenet and others, just not on the home network

Like I said on the other thread where you recently posted about that, DeejMaster1 :

Would you describe what problems you now experience, when trying to play H5 on a LAN ?

(In my case, as I have stated on another thread : version 2.01 runs fine on a LAN, but version 3.00 always crashes, when an A.I. turn gets stuck, after a few dozens of minutes.)