Contrary to people with blank minds, people with sufficiently loaded minds are able to get ideas from their thought repository whenever required. Is there an adjective or a noun for someone with a sufficiently loaded mind?
Often a description such as "quick-witted" or "thinks on his/her feet" is used for the case where a person is able to readily come up with an idea or response to a new situation.
He has a quick wit; he is never at a loss for words. (alternate: "a ready wit")
She can think on her feet; she easily comes up with new ideas on the fly.
You should be be very cautious about using the reverse, slow to describe someone's thinking process. However, it is okay to describe someone as a slow and patient or slow and careful thinker, if they are the sort of person who prefers to take their time considering the situation.
The most likely term I can think of is knowledgeable "intelligent and well informed".
EDIT: In response to the comment. Ah, so it's witty you want. That's odd, my mind is blank.
Well, how about erudite "having or showing great knowledge or learning"? Not witty, but less trivial.
The word "resourceful", when applied to people, means a person who can find correct and/or creative solutions to problems when required. For example:
Jack is a very resourceful engineer - even when the rest of the team is stuck, he can find a way to get things done.
That addresses the "able to get something acceptable out of your mind whenever you want" part you mentioned. There may be better words I could suggest if you could explain in more detail your concept of a "loaded mind". Note that that's an unusual expression, people would talk about a "loaded truck" or a shopping basked that's loaded with goods. When applied to people, "loaded" in colloquial speech actually means "rich":
Johnny is driving a brand new Porsche. His parents must be loaded.
If you're describing a well educated (or otherwise intelligent) individual, you could use 'intellectual'.
The new kid's very intellectual.
The new kid's an intellectual.
learnéd [(commonly written as “learned” and identified as an adjective from context)]
. . .(pronounced learn-ed) is an adjective meaning that the subject has done a lot of learning. It is more or less synonymous with "wise."
Source: answers.com, What is the difference between learned and learned?
well-informed or deeply versed through reading
Source: merriam-webster.com, definition of well-read
A person with a 'sufficiently loaded' mind (if not considered 'deranged!') would be considered to be:
A sharp-minded individual
...also someone with a 'sufficiently loaded mind' can be referred to as being on the ball, in gear or firing on all cylinders
If consistent, this person may be considered as bright.