David Mitchell is not entirely accurate here I think (but certainly more accurate then Mr missed-his-name).
Escalator steps are not meant to be walked on, see e.g. here:
http://www.theglobeandmail.com/globe-debate/that-stopped-escalator-its-a-tougher-climb/article623713/

So if you walk up a stationary escalator, the average person will have a lot harder time than on a regular staircase, so there will be less forward momentum at the top.

Mr missed-his-name would be Rhod Gilbert, a reasonably decent comedian from Wales (in case the accent didn't give it away).

http://youtu.be/G6afnyV8bf8 is him in action. Takes a while to get going, but the egg and cress sandwich stuff is good.

I would suppose that escalators have a bit higher steps than standard, which would also make it feel harder to scale than regular stairs.

Also toward the top steps get shorter as it begins straightening out and it confuses your brain/muscle memory compared to the regular stairs.

probably the strangest plug for a show i've ever heard

I don't often agree with Rhod Gilbert and I don't often disagree with David Mitchell, but it's not the same. The physics is the same but my mental expectation and muscle memory is different.

I usually take normal stairs two steps at a time, so the higher escalator steps (where I only take a single step at a time) actually are the lower ones for me. Would that mean that I get a higher momentum and speed at the top from walking up an escalator than ordinary stairs, then?