Assassins HAVE to kill in the blink of an eye
Think of an assassin IRL. Their target drops without a sound or a struggle, instantly. The entire reason assassins had been viable in competitive play was because they could INSTANTLY remove someone from the game, completely by themselves. A change from ".25 second burst to 1 second" will make assassins forever fail to reach competitive.
Keep the lone wolf+blink of an eye nature of assassinations. They reflect the assassin's identity, and the latter made them viable.