With respect, google maps is quite good at providing average walking times and it says 11 minutes for Vauxhall to Nine Elms. Most people can actually walk faster than googles times. I don't doubt that you struggle to keep up, indeed my mum would probably take twice as long given that she walks with at least a stick. However, the vast majority of people would cover the distance in that time and those that can't would know that they have to allow more.Good for you- it takes me a good 15 minutes just to walk from Vauxhall station to Embassy Gardens Waitrose.
Not everyone walks at super quick speed.