If you do it frequently over long periods of time, weed can make you slow. Not stupid, just slower in general. It takes you longer to think of things you want to say, longer to make decisions, etc. My friend Larry smoked weed nearly every day for a good year, now having a conversation with him feels like trying to dismantle a well built brick-wall. Slow, arduous, and pretty pointless unless you're under very specific circumstances.