I have to disagree. I think the two main scenes, such as the opening farmhouse scene and the scene in the bar, are good examples of how well-written dialogue scenes can build and create tension.
In both of those scenes they start out relatively calm and innocent. The fact that the initial dalogue may seem to be just general chit-chat and pleasantness hides the notion that behind it something could possibly happen. Then elements are introduced to add twists to the scenes, such as the gradual revelation that Waltz character actually knows that the farmer is hiding something, and in the bar scene the introduction of the german officer who joins the table. The tension is therefore racked up as the scene progresses.
Therefore I believe the scene needs to be a decent length to first of all lull the viewer into a false sense of security, then gradually introduce elements into the scene to grip the viewer.
I thought the dialogue in the farmhouse scene was good, (I didn't like the bar scene) but do I think it needed 20 mins worth of it? No. If they'd perhaps got it down to 10-13 mins it could have the same effect and the film would have not dragged so much