Among other things, it (most stimulants including caffeine and nicotine) constricts blood vessels, meaning less circulation to the brain.
Another side effect of quitting, lasts for weeks to months an increase in O2 levels in the blood. This may seem like a good thing, but if your body has been going for years on lower O2 levels, the sudden increase will literally fuck with your brain.
Or more circulation to the brain, depending on your blood pressure. Personally, I have low blood pressure, often to the extent I'll black out for a second or two when standing up from sitting/lying down for extended periods. Smoking with me, increases my blood pressure, so more blood to my brain, not to mention the increased heart rate.
As to the O2 levels, I can't say I know enough about that to respond, tp be honest.
Yes there are short term effects that might increase brain function for a very short period, but after that it's downhill, especially for long-time smokers.
Any perceived benefit of smoking should be outweighed by the fact that you're going to die early, the time you waste smoking (2-3 hours per pack, so for some people that can per day), and that there are better ways of increasing brain function, like changing what you eat.
Doesn't detract from the fact that, for a period of time after a cigarette, brain function will increase. Everyone knows the long term health issues, that's not a part of the argument.
Time wasted: Not if you're having a break anyway, or doing something else outdoors where you can smoke at the same time. Not to mention the fact that most people don't smoke a pack a day.
Also, as to changing what you eat, you can eat healthily and smoke, I know I do.
Not that I'm advocating smoking at all, just defending it a little.