Based solely off my high school friends who smoked and some are still smoking 25 years later and are the same morons they were back then. But then I realized they were always morons, never did much before they smoked and not much after so the marijuana likely just kept them in their comfort zone.
I have since met many engineers I work with and a vast majority have not only smoked it in college but use it in some form today as well and they are some of the sharpest people I know. My rep in Colorado and Northern California are almost exclusively recreational daily users and it’s replace their wine or beer at night with instead an edible after dinner and they are some of the top performers in their company.
My company tests for it so I’ve still never tried it but most of our engineers at corporate have all used or still use in some form despite the policy and have been fortunate to dodge random testing. I wouldn’t consider any of them less able to do their jobs because of it. Alcohol has cost two of our employees their jobs with DUIs and I much prefer working with marijuana users over functioning alcoholics any day.