I agree, weed has different effects on different people based on pre-existing personality traits and cognitive abilities. Different strains of weed also effects your perception differently, I personally find that indica makes me much less mentally active than a strong sativa, but a strong sativa while still changing certain aspects of my mental setting doesn't in it of itself ever put me in a more rational mindset. In fact I find that I'm much more susceptible to just accepting propositions at face value as opposed to thoroughly examining all of its facets. Different doses of course give you different effects, but once you pass a certain threshold you'll certainly not be very rational anymore :P Similarly with LSD, at a lower dose it's actually sort of sobering, you can think broader, faster, and more thoroughly, but there's certainly a difference between that and a much larger dose that would drive you directly into a psychosis.

THC tends to cause something called hyper priming which allows people to direct their attention towards specific ideas and goals that almost entirely correlate with your thinking at that time, though these can be more transient in some people, depending again on your personality. Some people while high are very prone to doing one thing for a long time (hyperpriming + distorted time perception) while others tend to move around much faster and not be able to settle on one specific thing (in my experience this is a bit rarer among users). This can be useful in a few ways imo, one it's a very good motivator and for some people it can even work similar to addreall to help them concentrate better on their work. I could smoke a bowl and watch a 2 hour lecture on quantum mechanics and be perfectly happy to sit there taking notes the entire time for some reason, but then ideas and judgement seem to be heavily impended after a while, which of course has an impact on your rationality. I'll be perfectly happy to accept most propositions when I'm high, and taking just a quick look around /r/psychonaut you can tell how easily delusional and conspiratorial type thinking is bolstered in some cases by psychedelics, and weed will certainly work in similar ways.