I don't do any sort of drug, or ever take medicine (unless I get real sick which doesn't happen often), so I can't really comment on my cubing ability while "high." I do expect though that those in this community that are actually intelligent, would benefit quite a bit from being high while cubing. If things really slow down, you could still focus your attention to the cube. You can put things into muscle memory, let that be executed just as quick as normal. If time truly slowed down, then I'd expect to see some ridiculous look ahead out of most of the sub20 people.
I'm not sure if it would work as well with pot though as it would with say coke. Pot is a downer, it is to make you more relaxed (generally a good thing in cubing). However, coke is an upper, so time slows down to you. Your reaction gets quicker. If your reaction gets better while on an upper (same sort of effect if you were to drink say two espresso's before cubing), then your look ahead should get ridiculously good as well, and your reaction to turning a face a wrong way should also come quicker than normal.
Basically I'm saying while high on pot you should be able to plan and be more efficient, and on coke you should be able to do the same and your reaction to something wrong should be higher as well. Unfortunately with pot your reaction will probably be slowed down.
Just my two cents.