Thorfin, the DND only works for zone messages. If you have 2 game rooms open and start a game in one of them, you still will receive all traffic coming from the other. So, if an idiot starts flooding the room you'll have problems.
But I've already noticed that the game lags more if it's in a crowded room like Urinal or if someone starts flooding, enven if I have just ONE room opened.
From my experience, I'd say that although the room window is frozen during a game, you still receive information from the Zone. After all, you receive all messages from the main chat when your game finishes, don't you? And I don't believe they were stored in the server waiting for your game to end...
If my feeling is correct and your results with NetMedic confirms it, you should start a game then alt-tab to close the main room window to experience less lag.
The sad point is that you have to return to the room to get other game...------------------
May your weapons be sharp and your ping be green