As far as the mental sharpness thing goes,I didn't notice any major changes. Some people I know would have some fogginess the day after. This never affected me personally. I think brain fog is definitely a real thing for _some_ users.
I totally understand this is anecdotal. I'm just trying to share my experiences. I hate how defensive people can be about cannabis. I am sympathetic to this because giving proponents of cannabis prohibition anything to latch onto delays progress. It should be legal. People should also have honest discussions about it. It's a fairly low risk drug IMO, but it is a drug.
On the plus side, I recently discovered low THC, high CBD strains that offer the anti anxiety benefits without loss of sharpness or short term memory. There’s almost no psychoactive effect, so this won’t get you high, but it will chill you out and help you get in the zone if you struggle with anxiety or PTSD symptoms.
Long after the high wears off, I sometimes consciously can tell "wow, I am really mentally underperforming today".
It's more difficult to form quick/sharp responses when talking, my though process isn't as sharp, and its kind of like my brain has more difficulty following trees of connections that I am normally very quick at.
After ending my five year exercise habit it became a lot harder to start exercising again, and brain fog and laziness began setting in. The highs also got a lot duller and less productive, and I subsequently began seeking the highs I knew from when I was in good health which led me down the wrong path for a while. I'm not blaming marijuana for this in any way, just acknowledging that I stopped exercising when I started smoking, and that some physical/mental health issues I've had since then most likely exist because I haven't been exercising.
In other words, putting psychoactive chemicals in your brain is a big deal and you should try to establish strong personal health and wellness routines before you begin doing it, and then prioritize those routines above the chemical habit.