Difficult decisions

When playing chess I often find myself losing myself in the jungle of variations. According to John Nunn's Secrets of Practical Chess, I should probably think less and move faster:

"When analysing a given position, it is fair to say that one almost always sees more in the first five minutes than in the next five minutes. The five minutes after that is even less productive and so on."

Nunn claims that "if a player spends more than 20 minutes over a move, the result is almost always a mistake." In his opinion, it is better to follow one's instinction or even just choose at random a move out of two equal possibilies than to analyse for too long.