To be clear - its not the amount of time it would take thats the main problem .Its HOW you stop the game, WHEN you stop the game, and the subsequent consequences of that, ie the stoppage of play, and the result if the video call is upheld or overruled.
Can we please accept that not EVERYTHING will be as instantly bloody obvious as the Lampard/Tevez incidents. There are any number of hugely controversial incidents that this would be called into play for to sort out, and its not always immediately obvious what the call should be.
Again it would ONLY be implemented when it IS obvious what the call should be, because the TV ref - who can see much better than the on pitch ref and is therefore a far far FAR better judge or correctness and fairness - deems it to be so.