The anonymous background artists at the Fleischer studio were called upon in several films to give the impression of speed during a pan. They would render part of their background blurry so it seemed the camera was moving quicker. Here’s an example from Parade of the Wooden Soldiers (1933), where the camera pans from right to left.
What’s really interesting about this sequence is the camera first pans from left to right, stops on the action, pans bottom to top, stops on the action, then does the right to left pan above, followed by a pan from bottom to top. I’d love to see what the whole background looked like. The camera eventually pulls back to reveal a full scene of the room, but a different background drawing was used when the camera begins to move out.
Here’s the centre part of the panned background a little later in the cartoon.