I do not know if this is valid for multiplayer lag, but does each of you run the game on a single CPU core? The game doesn't execute well on multiple CPU cores and this can cause quite some lag in singleplayer. With Windows Taskmanager assign the game's process to a single cpu core. Select the process, rightclick on it and then "Set affinity". Deselect "All Processors" and assign a specific CPU.
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