Basically, although base intelligence/smartness perhaps has two parameters that make it? Effort and speed. Everyone can put in a bit more effort, but base speed may be baked in, unless one trains it, and max reachable base speed will depend from person to person. Hell if I know, we haven't really created a definitive definition for intelligence yet.
Edit Addendum: As for what can be considered dumb or smart? I agree, lack of effort can be considered "dumb". Though the word dumb is a bit broad. I guess we can say many people are, out of habit, "intellectually heedless"