It might sound a bit arcane, but I attribute a lot of it to REM sleep. When I quit, I had crazy dreams every night, often nightmares. But frankly, I had dug myself into a pretty deep hole in the real world. These nightmares and unpleasant dreams became much less frequent over time, and I still dream almost every night. When I was smoking every day, I didn’t notice this. I don’t notice that was waking up foggy, it was nothing compared to a hangover or a night with little sleep. But now it’s clear. I feel like I have an entire dimension of sharpness and control I was missing and I’m grateful for that.
Overall depends what your goals are, not everybody constantly strives to be at their sharpest point, a bit of chill isn't bad from time to time. I think the biggest problem with MJ is that for most folks the best setup would be consuming from time to time, ie 1x a week. But its easy to slip into more frequent usage, where drawbacks become bigger and benefits smaller.
Let's find out more about you. How is your memory? What is your age? What does your history and current usage look like?