Slow Down Meaning

(idiomatic) Decelerate.

Example: 2013 June 1, “Ideas coming down the track”, The Economist, volume 407, number 8838, page 13 (Technology Quarterly):
  A “moving platform” scheme […] is more technologically ambitious than maglev trains even though it relies on conventional rails. […] This set-up solves several problems […]. Stopping high-speed trains wastes energy and time, so why not simply slow them down enough for a moving platform to pull alongside?
  When approaching a bend in the road, slow down, and speed up after leaving it.‎