#Improvisation, though incredibly effective, is a difficult skill to be used all the time.
The brain has to have full attention and focus to the problem, whose solution you need to find through improvisation, instantly remember [back] all the relevant information you know about the issue — and then relate them together, instantly!
To improvise on something, one can't be nervous; one has to be totally comfortable with the environment to consider all the factors. While this holds true for #planning as well, you get ample time to plan your strategy, which is not possible in improvisation.