There are some instances where walking can be faster but very specific:

With transits between Low (Numbered) B/High D (gates) and Low C/High A, using the secure airside elevated "Moving Sidewalks" is generally more efficient than the train.

But to the point; we once took 10 minutes to get from C38 to D25 after disembarking an hour late using the train (@C32 to @D20) and there was still a substantial amount of walking in that.