On Chris' theory...think of it this way...look at the shape of the tunnel. Stiff wires interspersed with gaps of fabric.
On the ground facing side, the variation in height of wire rib vs fabric on the ground is minimal. It's a fairly steady surface on which to run.
Now, on the sides without the ground limiting play, you get stiff wires with valleys of fabric in between, that can collapse akin to a varmint trap around a paw that goes it.
So, which do you think is faster? A compacted road with few scattered obstructions, or a pothole strewn road where the potholes try to eat you?
Not to say it *can't* be fast, say if your dog has mastered his stride to the point where he only walks on the wires, or depends on the centrifugal extension of the tunnel to provide the service surface.
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