Since no one cares to bring documentation I went and found a year old post that redirects to a reddit example of a client hack :
So now I learned that this is actually a thing which makes me mitigate my argument :
All depends of what you actually saw. Without hack, without exploit, it’s absolutely possible to have your animations extend from your rope time from the point of view of your opponent. All this hack is doing is emphasizing it.
The best example I saw of that was the tavern brawl blood magic when druids were able to play their whole deck on turn 1. Any player with good APM could do it, which resulted to a lot of games with absurdly long first turns
From your perspective you cannot really tell if your opponent simply played a lot of cards quickly at the end of your turn, or if they actually have a hacked client when it’s only a few couples of cards.
When it starts to get fishy is when they really started playing late in their turn and manage to play cards that they generated or drew reeeealy late in the animations. Cards a normal player couldn’t have had in hand in time to play them.