DECREASING THE WALKING SPEED

Make an event that's either parallel process or auto-start that decreases the movement speed of the hero once and then turns on a switch. Add a second page to this event that becomes active when this switch is on and keep it blank. Also make sure this event is set to "push key".

I would however suggest against decreasing the movement speed of a hero outside of cutscenes as nearly no one will want to play a game with this setting. Even when a game is designed to be small, this still gives the player a feeling of going nowhere slowly and will, in turn, keep them from enjoying a game as much as they could.

EDIT: Leaving the walking speed at the default also allows you to grow as a designer by not limiting yourself to designing the world around the speed you set for the player. A bonus to this is the ability to make larger, more elaborate dungeons and towns that encourage exploration, which is something all RPGs should attempt to express as much as possible as, most of the time, they're rarely anything more than a story book with battles (gameplay in RPGs is likely to be the most boring in terms of that "fun" factor, so any small touch of wonder does wonders).