Things like caltrop/ball bearings ref to moving at half speed how do you do that?

@PlaguescarredThings like caltrop/ball bearings ref to moving at half speed how do you do that? Treating each 5′ as 10 like difficult terain? yes, essentially pay an extra 1' per 1' moved

— Mike Mearls (@mikemearls) July 10, 2014