It is said, on an average Fast Bowlers take 5-6 min to bowl an Over. Whereas, Spinners take 4 - 5 avg min to bowl an over.
So, if we go by these numbers and your average of 20 secs to review a ball, then we do not need to set aside extra time to review each and every ball. As per Faf Du Plesis tweet, every ball can be reviewed in the background and a no ball communicated to Umpire through ear piece.
I am not fully in favor of the increase in use of technology in games where we have stopped enjoying the game with its flaws and imperfections. But I feel when technology is already widely used to review most of the game then why leave just few aspects to sole human judgement? Just a perspective to ponder upon and ensure a game’s outcome is fair and square.