Rather frequent use (as in not every day all the day but lets say couple of times a week) can have some decline but IMHO its still temporary. If the brain is already used to lower learning and complex analysis activity then it will be compounded but otherwise it shouldn't. I sometimes fall here, and I integrate internal banking systems for a living for 2+decades in very competitive place plus raising family and have mortgages, so no shortage of stress. Any miniscule decline is more than compensated by relief of stress and some giggly happiness, plus that massive creativity spike which allows me to catch up with stuff slowly sliding out of focus (and then biting hard like bureaucracy, taxes etc). Literally subconsciousness sliding notes underneath the doors of my mind, topics often pop out of blue, in such frequency that I sometimes struggle to keep up and take notes since next one with higher priority comes and then next one.

Full on stoner - thats a goner, if we talk decades of use, I think. YMMV. But - I've met folks who had periods of life where they smoked all the time every day, for 5-10 years, usually teens (where I would say folks are very vulnerable since brain is still evolving). All are fine now in various roles in their lives.