Alcohol is a central nervous system depressant, it slows things down. So does morphine, heroin, codeine, etc. This can be beneficial in playing and the creative process, but only to some extent and only in some situations. How do you think Amy Winehouse would have sounded if she wasn't on her depressant of choice? I would argue that she would not have had that sultry blues feel when totally straight.
THC (active ingredient in the Devil's weed) is a central nervous system stimulant. Think speed! Now many people will say "Oh, smoking pot makes me tired". This is true for many, but you can not blame the active ingredient for that tired feeling, what you need to blame there is filling your lungs with tar (20 times the tar in marijuana than there is in tobacco so smoking a joint is equivalent tar wise to smoking a full pack of cigs), which reduces the oxygen that is absorbed by your lungs, slowing your brain down due to that reduced oxygen flow. Try eating some edibles and you will understand what just the THC does when you don't have the tar clogged lung effects!
I would agree that moderation is prudent in all intakes when performing, and as DTR said, if you are trying to shred or play something in a 7/3 time signature, the more you consume the less likely you are going to nail it (although it may sound great in your mind at that time). It can however give you that slowhand vibe, or sultry voice, etc. when controlled properly, but as your state of mind changes, so does your perception so it can be difficult!
Now in the creative process, alcohol has the consequence of removing the "filters" that your sober mind imposes. That is why when drunk, people will say exactly what they are thinking rather than filtering it. The alcohol did not create those thoughts, they were already in that persons mind, but having the sober filters removed, it comes out often without thinking about the consequences or how it will affect others. So creatively, less filters can be a good thing, but could also produce a higher concentration of gibberish! THC, in my case anyway, stimulates the creative process, as does Psilocybe Cubensis! Unlike the "filters removed" consequence of alcohol, with these other substances, my mind climes trees that I did not even know were there in my "sober" state, so it can be very rewarding to discover and develop those hidden gems. I guess you could say it is another kind of filter being removed, but more of the type that allows you to access hidden items.
I used to buy tequila by the case, and that case would often not even last a month, often drinking a full 750ml myself in an evening! Glad I was not playing during those heavy consumption phases, the suffering I would have put people through is unbearable to even imagine! Those days are decades gone, but I still love tequila and beer, although these days 3-6 is my limit, regardless of what I am doing!
For decades I have told people "Moderation in all things, including moderation"! Sometimes (very rarely) I still like to exceed the boundaries, but it is at best once or twice a year and only when I know I am "safe" to cross the boundaries! So take it easy, but let yourself loose when the time is right, regardless of the intake! Likely the best time will not be at performance time, but maybe afterwards!
Happy and safe substance consumption to all!!!