That's their choice though. Are you really going to blame a plant for their becoming slow and lazy versus their decision to use it? Furthermore, I'd just point out that people who seem bright and happy on the outside can actually be suffering on the inside. It's possible they're happier and "brighter" now, just not in the same metric you were measuring them by.

(Note: I don't have a horse in this race- I don't use marijuana because I don't like the effects, but I'll be the last person to act like it's some demon plant out to hurt capitalism's productivity.)