I experienced the same when I visited NYC in March. It was enough to put me off going back until they ban it in public spaces.
I've no issue with people smoking weed in their own homes but the stench was so overwhelming in places like Times Square that I felt sick. There seemed to be far more weed smoking than simple cigarette smoking!
The other thing I noticed about NYC was just how bl**dy expensive food was. Far pricier than London.