I think it would be good (fair) if the mulligan-animation was only played once both players clicked either “Keep” or “Redraw”. Reasons ->
I just wrote in my guide:
“If you plan to mulligan (draw a new opening hand), try to not do it before your opponent! Keep an eye on your opponents hand, weather or not she mulligans is important information. If she does mulligan there is a chance that she has a bad hand and you can punish it by being aggressive. That is why you should never mulligan before your opponent (you either do it after them or in the last second).”
It is simple to get a small advantage by doing this, and sooner or later everyone will do it. Which means everyone will wait 15 seconds before clicking “redraw”, which adds 5-10 unneccesary seconds per game.