> I would speculate that LSD turns on System 2 where normally my System 1 brain would give me an immediate response.
As far as I've read, LSD is, surprisingly, just a serotonin agonist, similar to anti-depressants or MDMA. (Technically, it's a non-selective serotonin agonist, and it does this by reversing the chemical reactions that take serotonin apart, to basically "un-use-up" serotonin you've already spent--which might explain its differing experiential effects--but in the end it still means "more serotonin in the brain.")
Now, the two things serotonin has been heavily implicated for in neurology, are pleasure (though not reward; that's dopamine), and neurogenesis: serotonin is basically released when something "feels good" in a sensual way--something looks aesthetically pleasing, something tastes delicious, etc.--as a signal to the rest of your brain to backtrack to what caused this stimulus to happen, and build up the neurons involved. Basically, if I find some tasty berries while foraging, reinforce the memory of the walk I took to get to the berries. [This effect can be harnessed: consume a bit of some fat-saturated food right after you study something, if you want to remember it better.]
But LSD, MDMA, and other drugs that stimulate serotonin release, can have psychedelic effects well beyond what you'd expect from "remembering things better." This is because the reaction to serotonin, neurogenesis, goes way beyond sensible memory formation. It takes any spikes in electrochemical activity that are going on in the brain at the time, and says "that: build up the synapse for that." And, of course, neurons have an underlying level of noise, that usually gets buried under actual cognition--so, when you turn the "gain" up on what "becomes brain cells", suddenly you start forming 'concepts' for ideas that are made of nothing at all--the feeling of seeing shared properties between things that have none. Which would indeed, likely, be best described as "a trip."
Of course, this also informs the other thing people--especially those in creative fields--say about LSD: that it's a life-altering experience, quite foundational for later productive work. This is basically because pushing up the "noise floor," if you have a lot of potential ideas lurking just below the surface as potential connections you haven't made between concepts, ready to be exploited--they'll get wired together by the non-selective flood of serotonin too. But, unlike the nodes for "the purpleness of music" or what-have-you, these nodes will actually self-reinforce once created--that is, they're useful to have, so you'll keep using them--so they'll stick around, whereas the other synapses will just get their reactivity scaled back down when it turns out how purple a song is doesn't have any causal impact on anything else. :)