I really don't see where there is variable speed in that demo. It looks more like slow downs. There is also the possibility that the speed Sam was walking in that passage was a part of an animation. Like you tell something Sam should do, he will go there and do it. If you point at the weapon to pick it up, he will walk to the weapon and pick it up. That's happening in one animation and the speed in which Sam is walking in this animation may be different to the normal walking you do yourself.
Variable Speed is really an important aspect. It doesn't just add a lot of depth to the game and control to the character, it's also the best feature to blend the Panther and Ghost player. We have a difficulty system, make sure you use that system to your benefit. You can change a lot of things there. In previous games the audio and L&S system was also partly linked to the difficulty or rather the AI having more sharp senses (better eyes and ears).
Both the Panther and Ghost player benefit from that system and it's in my opinion a great feature especially for the Panther player (Aggressive Panthers and the more Defensive ones) because they will be the one who will be engaging the enemies in an active way. They need to have the ability to stalk their prey, their enemy. Variable Speed gives maximum controll, maximum depth and maximum range of playstyles blending them in perfectly. It needs to be there.