When you keep your posture up right, you will naturally move slower.
Try this experiment:
Wear your normal casual clothes with trainers etc, and analyse your natural movement speed.
Then, change into a business suit and wear leather shoes. Look in the mirror and see how cool and Boss like you are... and start walking...
You will notice that you will naturally be walking slow with shoulders back and chest up.
- This is the pace you want, and it will be natural.